Make OPTIONS method on MSC3916 endpoints available without auth (#3431)

OPTIONS method is usually sent by browser in preflight requests, most of
the time we cannot control preflight request to add auth header.

Synapse will return a 204 response directly without authentication for
those OPTIONS method.

According to firefox's documentation, both 200 and 204 are acceptable so
I think there is no need to change handler in dendrite.

This closes https://github.com/matrix-org/dendrite/issues/3424

No need to add a test because this is just a fix and I have tested on my
Cinny Web client personally.

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