build: delete/replace clang warning pragmas

- delete redundant warning suppressions for `-Wformat-nonliteral`.
  This now relies on `CURL_PRINTF()` and it's theoratically possible
  that this macro isn't active but the warning is. We're ignoring this
  as a corner-case here.

- replace two pragmas with code changes to avoid the warnings.

Follow-up to aee4ebe591 #12803
Follow-up to 0923012758 #12540
Follow-up to 3829759bd0 #12489

Reviewed-by: Daniel Stenberg
Closes #12812

lib/memdebug.c

@@ -448,14 +448,7 @@ void curl_dbg_log(const char *format, ...)
     return;
 
   va_start(ap, format);
-#ifdef __clang__
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wformat-nonliteral"
-#endif
   nchars = mvsnprintf(buf, LOGLINE_BUFSIZE, format, ap);
-#ifdef __clang__
-#pragma clang diagnostic pop
-#endif
   va_end(ap);
 
   if(nchars > LOGLINE_BUFSIZE - 1)