MINOR: ssl/ocsp: Remove the useless res_payload callback of the http-client

MINOR: ssl/ocsp: Remove the useless res_payload callback of the http-client

The OCSP update task does not process the response payload on the fly. The
whole payload is accumulated in the http-client response buffer and only
retrieved when the response is fully received, on the res_end callback. So
there is no reason to define a res_payload callback. It was only used to set
the HC_OCSP_RES_BODY flag, to know if a payload was received or not. Testing
the content of the response buffer is enough to do so.

The res_payload callback and the HC_OCSP_RES_BODY flag are thus removed.
This way, the OCSP update task now works the same way than the ACME one.