fscrypt: remove fscrypt_fname_usr_to_disk()

fscrypt_fname_usr_to_disk() sounded very generic but was actually only
used to encrypt symlinks.  Remove it now that all filesystems have been
switched over to fscrypt_encrypt_symlink().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>