Remove old @Postsubmit annotation

The annotation is no longer needed because it has been replaced with
@FlakyTest. This commit also removes the import.

Change-Id: I5e7c1a7b6a856f84f294f98acb4ba627fc7880cd
Merged-In: I5e7c1a7b6a856f84f294f98acb4ba627fc7880cd
Fixes: 35710737
Test: No change to behavior