ext4: stop calling fscrypt_add_test_dummy_key()

Now that fs/crypto/ adds the test dummy encryption key on-demand when
it's needed, there's no need for individual filesystems to call
fscrypt_add_test_dummy_key().  Remove the call to it from ext4.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230208062107.199831-3-ebiggers@kernel.org