Update orml-unknown-tokens unit test

[shaunxw]

With paritytech/substrate#8407, the DispatchError equality comparison workaround is not needed any more.

open-runtime-module-library/unknown-tokens/src/tests.rs

Lines 36 to 38 in 523c95a

	fn convert_err(err: DispatchError) -> DispatchError {
	DispatchError::decode(&mut &err.encode()[..]).expect("encode then decode cannot fail")
	}