Revert "Fallback to stop-the-world compaction on kernel < 5.13 hosts"

This reverts commit aac790f8ffe107c24f2487e93ab6f5b5598c4419.

Reason for revert: Not working as intended. The userfaultfd GC is still being chosen even if kernel version is < 5.13 (b/242181443).
Test: presubmit

Change-Id: I65a05ed44539e5629f4b4f584823cca5e3ca36a9
1 file changed