mnt_idmapping: remove nop check

All mounts default to nop_mnt_idmap and we don't allow creating idmapped
mounts that reuse the idmapping of the filesystem. So unless someone
passes a non-superblock namespace to these helpers this check will
always be false. Remove it and replace it with a simple check for
nop_mnt_idmap.

Link: https://lore.kernel.org/r/20231122-vfs-mnt_idmap-v1-2-dae4abdde5bd@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>