security: Kill off the sb_copy_data hook

Kill off the sb_copy_data hook as it's now unnecessary since the non-binary
mountdata legacy filesystems have their arguments chopped up, passed
through the new normal parameter interface, where LSM arguments are
removed, and then reassembled to be passed to ->mount().

So by the time the security_sb_copy_data() is called by legacy_validate(),
there are no LSM options left to process.

Signed-off-by: David Howells <dhowells@redhat.com>