cleanup: remove the 'numsocks' argument used in many places

It was used (intended) to pass in the size of the 'socks' array that is
also passed to these functions, but was rarely actually checked/used and
the array is defined to a fixed size of MAX_SOCKSPEREASYHANDLE entries
that should be used instead.

Closes #4169

lib/transfer.h

@@ -47,8 +47,7 @@ CURLcode Curl_readwrite(struct connectdata *conn,
                         struct Curl_easy *data, bool *done,
                         bool *comeback);
 int Curl_single_getsock(const struct connectdata *conn,
-                        curl_socket_t *socks,
-                        int numsocks);
+                        curl_socket_t *socks);
 CURLcode Curl_readrewind(struct connectdata *conn);
 CURLcode Curl_fillreadbuffer(struct connectdata *conn, size_t bytes,
                              size_t *nreadp);