HDFS-16647.Delete unused NameNode#FS_HDFS_IMPL_KEY.

[jianghuazhu]

Description of PR

It looks like NameNode#FS_HDFS_IMPL_KEY is not being used anywhere, it would be cleaner to remove it.
Details: HDFS-16647

How was this patch tested?

Here is just to delete some unused code, it is not too stressful for testing.

[ZanderXu]

LGTM

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 37s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+0 🆗	detsecrets	0m 0s		detect-secrets was not available.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
-1 ❌	test4tests	0m 0s		The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
			_ trunk Compile Tests _
+1 💚	mvninstall	38m 10s		trunk passed
+1 💚	compile	1m 46s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	compile	1m 38s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	checkstyle	1m 20s		trunk passed
+1 💚	mvnsite	1m 46s		trunk passed
+1 💚	javadoc	1m 21s		trunk passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	1m 43s		trunk passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	3m 36s		trunk passed
+1 💚	shadedclient	23m 36s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	1m 18s		the patch passed
+1 💚	compile	1m 23s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javac	1m 23s		the patch passed
+1 💚	compile	1m 18s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	javac	1m 18s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	checkstyle	0m 56s		the patch passed
+1 💚	mvnsite	1m 21s		the patch passed
+1 💚	javadoc	1m 1s		the patch passed with JDK Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1
+1 💚	javadoc	1m 30s		the patch passed with JDK Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
+1 💚	spotbugs	3m 31s		the patch passed
+1 💚	shadedclient	23m 4s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
-1 ❌	unit	246m 28s	/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt	hadoop-hdfs in the patch passed.
+1 💚	asflicense	1m 14s		The patch does not generate ASF License warnings.
		356m 26s

Reason	Tests
Failed junit tests	hadoop.hdfs.server.namenode.ha.TestObserverNode
	hadoop.hdfs.TestRollingUpgrade

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4525/1/artifact/out/Dockerfile
GITHUB PR	#4525
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient spotbugs checkstyle codespell detsecrets
uname	Linux c3b0104732ed 4.15.0-175-generic #184-Ubuntu SMP Thu Mar 24 17:48:36 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 7a62423
Default Java	Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Multi-JDK versions	/usr/lib/jvm/java-11-openjdk-amd64:Private Build-11.0.15+10-Ubuntu-0ubuntu0.20.04.1 /usr/lib/jvm/java-8-openjdk-amd64:Private Build-1.8.0_312-8u312-b07-0ubuntu1~20.04-b07
Test Results	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4525/1/testReport/
Max. process+thread count	3519 (vs. ulimit of 5500)
modules	C: hadoop-hdfs-project/hadoop-hdfs U: hadoop-hdfs-project/hadoop-hdfs
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4525/1/console
versions	git=2.25.1 maven=3.6.3 spotbugs=4.2.2
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[ayushtkn]

LGTM

[jianghuazhu]

Thanks @ayushtkn and @ZanderXu .