KAFKA-19451: fix flaky test RemoteIndexCacheTest.testCacheEntryIsDeletedOnRemoval()

[DL1231]

Problem Description
In the RemoteIndexCache.cleanup() method, the asynchronous invocation of index.deleteIfExists() may cause a conflict. When the getIndexFileFromRemoteCacheDir() method is executed, it utilizes Files.walk() to traverse all files in the directory path. If index.deleteIfExists() is triggered during this traversal, a NoSuchFileException will be thrown.

Solution
To resolve this issue, ensure that index.deleteIfExists() has been fully executed before invoking getIndexFileFromRemoteCacheDir().