Revert "ANDROID: user: Add vendor hook to user for GKI purpose"

This reverts commit c9b8fa64.

The hooks android_vh_alloc_uid and android_vh_free_user are not used by
any vendor, so remove them 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: 187458531
Cc: heshuai1 <heshuai1@xiaomi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I2aa3f95ab52c2d81eb790ba8b64b864f8189222b