daphne_worker: DapGlobalConfig.supported_hpke_kems has no affect

[cjpatton]

Previously this was used to pick the KEM algorithm to when generating a fresh HPKE receiver config to populate KV. As of #353 the HPKE config is expected to be configured manually. Moreover the preference is determined by the order of the configs in KV.

We should just remove this field, since it no longer impacts anything.