cf-socket: make socket data_pending a nop

Eliminating the socket readability check in the socket connection
filters for the 'data_pending' callback. Improves performance of
handling of transfers, up to ~30%, depending on parallelism and response
size.

Whatever `data_pending()` once was, its semantics are now:
"Is there anything buffered in the connection filters that needs
 receive?"
Any checks of the socket's readability are done via `multi_wait()`
and friends.

Fix the one place in HTTP/1 proxy code that checked `data_pending()` and
did an early return if false. Remove that check and actually try to
receive data every time.

Closes #17785

lib/cf-h1-proxy.c

@@ -374,9 +374,6 @@ static CURLcode recv_CONNECT_resp(struct Curl_cfilter *cf,
   error = SELECT_OK;
   *done = FALSE;
 
-  if(!Curl_conn_data_pending(data, cf->sockindex))
-    return CURLE_OK;
-
   while(ts->keepon) {
     size_t nread;
     char byte;