Clean up and adjust USB mode

Change-Id: If6e2873ddf1364d87a3a7aa4d8c88373eca5ebc3
Remove USB mass_storage setting from device init

USB mass_storage mode is now set and unset automatically in the default
TWRP init.rc.

Change-Id: Iae752005c1153c3853c4fa9dd3531b53de6df6d8