MSC2713: Remove deprecated Identity Service endpoints

Implementations will have had plenty of time to adopt the new v2 API for Identity Servers, so we should clean out the old endpoints.

All deprecated endpoints in the r0.3.0 Identity Service API specification are to be removed.

For completeness, this includes:

GET /_matrix/identity/api/v1
GET /_matrix/identity/api/v1/pubkey/{keyId}
GET /_matrix/identity/api/v1/pubkey/isvalid
GET /_matrix/identity/api/v1/pubkey/ephemeral/isvalid
GET /_matrix/identity/api/v1/lookup
POST /_matrix/identity/api/v1/bulk_lookup
POST /_matrix/identity/api/v1/validate/email/requestToken
POST /_matrix/identity/api/v1/validate/email/submitToken
GET /_matrix/identity/api/v1/validate/email/submitToken
POST /_matrix/identity/api/v1/validate/msisdn/requestToken
POST /_matrix/identity/api/v1/validate/msisdn/submitToken
GET /_matrix/identity/api/v1/validate/msisdn/submitToken
GET /_matrix/identity/api/v1/3pid/getValidated3pid
POST /_matrix/identity/api/v1/3pid/bind
POST /_matrix/identity/api/v1/3pid/unbind
POST /_matrix/identity/api/v1/store-invite
POST /_matrix/identity/api/v1/sign-ed25519