Removed unused mount mode constants

Since FUSE in R, there are only 4 mount modes that are used
from IVold.aidl:

REMOUNT_MODE_NONE
REMOUNT_MODE_DEFAULT
REMOUNT_MODE_INSTALLER
REMOUNT_MODE_PASS_THROUGH
REMOUNT_MODE_ANDROID_WRITABLE

Bug: 148454884
Test: Manual
Change-Id: I62b66083f2099b429474872b6f9c7cffb9fb906d