Revert "ANDROID: mm: add vendor hook for vmpressure"

This reverts commit 444a0b77.

The hook android_vh_vmpressure is not used by any vendor, so remove it
to help with merge issues with future LTS releases.

If this is needed by any real user, it can easily be reverted to add it
back and then the symbol should be added to the abi list at the same
time to prevent it from being removed again later.

Bug: 203756332
Bug: 191534577
Cc: Liangliang Li <liliangliang@vivo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I63d2a210dc6a6ded20ea8779af719d70f368ce0f