remount: Remove all remaining opaque exit code (RemountStatus enum)

All errors are already logged and the exit code is not used anywhere by
anyone.
Functions should instead return (true/false) or
(EXIT_SUCCESS/EXIT_FAILURE) to indicate error state, and log error
reasons to logd and stderr.

Bug: 241688845
Test: adb-remount-test
Change-Id: Iba86a814a75f81ed0f6e43659d1aca72813824bc