headers: Remove no longer exported functions

There were a leftover few prototypes of Curl_ functions that we used to
export but no longer do, this removes those prototypes and cleans up any
comments still referring to them.

Curl_write32_le(), Curl_strcpy_url(), Curl_strlen_url(), Curl_up_free()
Curl_concat_url(), Curl_detach_connnection(), Curl_http_setup_conn()
were made static in 05b100aee2.
Curl_http_perhapsrewind() made static in 574aecee20.

For the remainder, I didn't trawl the Git logs hard enough to capture
their exact time of deletion, but they were all gone: Curl_splayprint(),
Curl_http2_send_request(), Curl_global_host_cache_dtor(),
Curl_scan_cache_used(), Curl_hostcache_destroy(), Curl_second_connect(),
Curl_http_auth_stage() and Curl_close_connections().

Closes #4096
Reviewed-by: Daniel Stenberg <daniel@haxx.se>

lib/http.c

@@ -383,7 +383,7 @@ static bool pickoneauth(struct auth *pick, unsigned long mask)
 }
 
 /*
- * Curl_http_perhapsrewind()
+ * http_perhapsrewind()
  *
  * If we are doing POST or PUT {
  *   If we have more data to send {