fs: Remove internal old mount API code

fs: Remove internal old mount API code

Now that the last in-tree filesystem has been converted to the new mount
API, remove all legacy mount API code designed to handle un-converted
filesystems, and remove associated documentation as well.

(The code to handle the legacy mount(2) syscall from userspace is still
in place, of course.)

Tested with an allmodconfig build on x86_64, and a sanity check of an
old mount(2) syscall mount.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://patch.msgid.link/20251212174403.2882183-1-sandeen@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>