dm: remove special-casing of bio-based immutable singleton target on NVMe

Commit 9c37de29 upstream.

There is no benefit to DM special-casing NVMe. Remove all code used to
establish DM_TYPE_NVME_BIO_BASED.

Also, remove 3 'struct mapped_device *md' variables in __map_bio() which
masked the same variable that is available within __map_bio()'s scope.

Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>