Fix KeyError on unbinding addresses

[anoadragon453]

There was a typo in the removeBinding function which seems to be causing unbindings to fail in Sydent.

The function normalise_address expects the medium to be passed as the second argument:

sydent/sydent/util/stringutils.py

Lines 131 to 135 in d6db3b1

	def normalise_address(address, medium):
	if medium == "email":
	return address.casefold()
	else:
	return address

Which in most cases would be "email". The code had threepid["email"] instead.

Introduced in #374.

[clokep]

Smells like we don't have tests for unbinding of addresses?

[anoadragon453]

@clokep I think you may be correct! One should probably live in https://github.com/matrix-org/sydent/blob/main/tests/test_email.py.

I'll create a separate issue for adding some.