use item.isDeleted to check whether the item is deleted

[nobodyiam]

What's the purpose of this PR

Use item.isDeleted to check whether the item is deleted, as item.dataChangeLastModifiedBy is not null for deleted items since #2680.

Which issue(s) this PR fixes:

Fixes #4276

Brief changelog

• Use item.isDeleted to check whether the item is deleted

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit tests to verify the code.
• Run mvn clean test to make sure this pull request doesn't break anything.
• Update the CHANGES log.

[codecov-commenter]

Codecov Report

Merging #4279 (bb0d07d) into master (8d623d5) will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4279      +/-   ##
============================================
+ Coverage     52.56%   52.59%   +0.03%     
- Complexity     2641     2642       +1     
============================================
  Files           488      488              
  Lines         15253    15253              
  Branches       1577     1577              
============================================
+ Hits           8017     8022       +5     
+ Misses         6685     6681       -4     
+ Partials        551      550       -1     

Impacted Files	Coverage Δ
...ework/apollo/internals/RemoteConfigRepository.java	88.34% <0.00%> (+3.06%)	⬆️

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[Anilople]

LGTM