Revert "gdatetime: Enable compile time check of g_date_time_format() format"

This reverts commits:
 • 9ddcc79502
 • ae02adc3c3

g_date_time_format() supports a few non-standard format placeholders:
 • %:z
 • %::z
 • %:::z

These are all gnulib strtime() extensions, and hence are not recognised
by the compiler when the function is annotated with G_GNUC_STRFTIME.
However, this wasn’t noticed when we originally merged this change
because the errors were disabled in the tests which covered those
placeholders.

tests/datetime.c

@@ -18,14 +18,6 @@
 #include <locale.h>
 #include <glib/glib.h>
 
-/* The whole purpose of this test is to pass the command line argument
- * to g_date_time_format(), so the format argument cannot be compile-
- * time checked. Disable the inevitable compiler warning.
- */
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-Wformat-nonliteral"
-#endif
-
 int
 main (int argc, char *argv[])
 {