Remove signature check on v1 identity server lookups (#8001)

We've [decided](https://github.com/matrix-org/synapse/issues/5253#issuecomment-665976308) to remove the signature check for v1 lookups.

The signature check has been removed in v2 lookups. v1 lookups are currently deprecated. As mentioned in the above linked issue, this verification was causing deployments for the vector.im and matrix.org IS deployments, and this change is the simplest solution, without being unjustified.

Implementations are encouraged to use the v2 lookup API as it has [increased privacy benefits](https://github.com/matrix-org/matrix-doc/pull/2134).