MINOR: Remove DeleteSegmentsByRetentionTimeTest#executeTieredStorageTest flaky annotation (#19301)

This test was fixed by [this
commit](#18861) and hasn't failed
for about two weeks. Therefore, we can remove the `@Flaky` annotation.

Gradle report:
https://develocity.apache.org/scans/tests?search.rootProjectNames=kafka&search.startTimeMax=1743081652853&search.startTimeMin=1741795200000&search.tags=github%2Ctrunk&search.timeZoneId=Asia%2FTaipei&tests.container=org.apache.kafka.tiered.storage.integration.DeleteSegmentsByRetentionTimeTest

Reviewers: TaiJuWu <[email protected]>, Chia-Ping Tsai
<[email protected]>

Author: m1a2st

storage/src/test/java/org/apache/kafka/tiered/storage/integration/DeleteSegmentsByRetentionTimeTest.java

@@ -17,11 +17,9 @@
 package org.apache.kafka.tiered.storage.integration;
 
 import org.apache.kafka.common.config.TopicConfig;
-import org.apache.kafka.common.test.api.Flaky;
 
 import java.util.Map;
 
-@Flaky("KAFKA-18606")
 public final class DeleteSegmentsByRetentionTimeTest extends BaseDeleteSegmentsTest {
 
     @Override