Remove mtd_device type.

mtd_device does not label any /dev node present on walleye, and the only
permission to that type is:
allow hal_telephony_server mtd_device:dir search;
I suspect there is no need to keep mtd_device around.

Bug: 110962171
Test: boot aosp_walleye
Change-Id: If74b1258b21edeca38c8b7dc07a3a10b751a7e85