fs: remove unused low-level mapping helpers

Now that we ported all places to use the new low-level mapping helpers
that are able to support filesystems mounted with an idmapping we can
remove the old low-level mapping helpers. With the removal of these old
helpers we also conclude the renaming of the mapping helpers we started
in commit a65e58e7

 ("fs: document and rename fsid helpers").

Link: https://lore.kernel.org/r/20211123114227.3124056-8-brauner@kernel.org (v1)
Link: https://lore.kernel.org/r/20211130121032.3753852-8-brauner@kernel.org (v2)
Link: https://lore.kernel.org/r/20211203111707.3901969-8-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>