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

Since commit 5a6c35f9 ("block: remove direct_make_request") there
is no benefit to DM special-casing NVMe. Remove all code used to
establish DM_TYPE_NVME_BIO_BASED.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>