blockdev: Drop unused drive_get_next()

drive_get_next() is basically a bad idea.  It returns the "next" block
backend of a certain interface type.  "Next" means bus=0,unit=N, where
subsequent calls count N up from zero, per interface type.

This lets you define unit numbers implicitly by execution order.  If the
order changes, or new calls appear "in the middle", unit numbers change.
ABI break.  Hard to spot in review.

The previous commits eliminated all uses.  Drop the function.

Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211117163409.3587705-14-armbru@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>