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: Id70e5a5d08e5de79f391e19ea97e356f39a3ed51