uncrypt: drop generic block_device allow rules

Currently, uncrypt has write access to "block_device". This is
the generic label used for a file in /dev/block which doesn't
have a more specific label assigned to it.

This is an overly broad grant. Commit a10f789d
started the process of deprecating "block_device" access in favor
of "misc_block_device".

This change completes the deprecation and removes the overly
broad grant. Also update the neverallow rules so that
this overly broad rule cannot be reintroduced into uncrypt.

Bug: 25091603
Change-Id: Ifc5fa412db2f95726ae89c32c577a6659885ae55