block: Drop raw_refresh_limits()

The raw block driver was blindly copying all limits from bs->file,
even though: 1. the main bdrv_refresh_limits() already does this
for many of the limits, and 2. blindly copying from the children
can weaken any stricter limits that were already inherited from
the backing chain during the main bdrv_refresh_limits().  Also,
a future patch is about to move .request_alignment into
BlockLimits, and that is a limit that should NOT be copied from
other layers in the BDS chain.

Thus, we can completely drop raw_refresh_limits(), and rely on
the block layer setting up the proper limits.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
