Revert "mm: protect VMA modifications using VMA sequence count"

This reverts commit 06219e6c.

Bug: 128240262
Change-Id: If31a4c81badd891e6ca5740dbd022b5edbe47254
Signed-off-by: Minchan Kim <minchan@google.com>
[dereference23: Forward port to msm-5.4]
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>