[SPARK-28199][SS][FOLLOWUP] Remove unnecessary annotations for private API

## What changes were proposed in this pull request?

SPARK-28199 (apache#24996) hid implementations of Triggers into `private[sql]` and encourage end users to use `Trigger.xxx` methods instead.

As I got some post review comment on apache@7548a88#r34366934 we could remove annotations which are meant to be used with public API.

## How was this patch tested?

N/A

Closes apache#25200 from HeartSaVioR/SPARK-28199-FOLLOWUP.

Authored-by: Jungtaek Lim (HeartSaVioR) <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: HeartSaVioR

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/Triggers.scala

@@ -21,7 +21,6 @@ import java.util.concurrent.TimeUnit
 
 import scala.concurrent.duration.Duration
 
-import org.apache.spark.annotation.{Evolving, Experimental}
 import org.apache.spark.sql.streaming.Trigger
 import org.apache.spark.unsafe.types.CalendarInterval
 
@@ -47,15 +46,12 @@ private object Triggers {
  * A [[Trigger]] that processes only one batch of data in a streaming query then terminates
  * the query.
  */
-@Experimental
-@Evolving
 private[sql] case object OneTimeTrigger extends Trigger
 
 /**
  * A [[Trigger]] that runs a query periodically based on the processing time. If `interval` is 0,
  * the query will run as fast as possible.
  */
-@Evolving
 private[sql] case class ProcessingTimeTrigger(intervalMs: Long) extends Trigger {
   Triggers.validate(intervalMs)
 }
@@ -84,7 +80,6 @@ private[sql] object ProcessingTimeTrigger {
  * A [[Trigger]] that continuously processes streaming data, asynchronously checkpointing at
  * the specified interval.
  */
-@Evolving
 private[sql] case class ContinuousTrigger(intervalMs: Long) extends Trigger {
   Triggers.validate(intervalMs)
 }