libprocessgroup: Remove MemcgV2Test#ProactiveReclaimDoesntTakeForever

Post-boot CPU activity is interfering with this VTS test, causing the
20s threshold to be exceeded on some devices. The platform does not
(yet) use root memory.reclaim, so remove the test for now. If it's
brought back, we need to add a strategy to wait until post-boot CPU
activity has declined before reclaiming.

Bug: 389749646
Test: atest vts_libprocessgroup
Change-Id: I0a185db96591a22eb7377ae7866135c99c0339ad