Revert "ANDROID: mm: export zone_watermark_ok"

This reverts commit e909fe79.

The symbols exported in it were never used by any external modules, so
remove the exports as they are not needed.

Bug: 203756332
Bug: 140294230
Cc: Sudarshan Rajagopalan <sudaraja@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ib3315ba129773256398a71ffe4c3acb55075b012