fscrypt: remove fscrypt_set_test_dummy_encryption()

Now that all its callers have been converted to
fscrypt_parse_test_dummy_encryption() and fscrypt_add_test_dummy_key()
instead, fscrypt_set_test_dummy_encryption() can be removed.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20220513231605.175121-6-ebiggers@kernel.org