ANDROID: Incremental fs: Small improvements

Rmove bc_mutex used to protect metadata chain, now that is only
read at file open time
Remove certain unused mount options

Bug: 172482559
Test: incfs_test passes
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I31db6316876086bfbcec3ecdd400ef74688d556d