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.

glib/gdatetime.h

@@ -261,7 +261,7 @@ GDateTime *             g_date_time_to_utc                              (GDateTi
 
 GLIB_AVAILABLE_IN_ALL
 gchar *                 g_date_time_format                              (GDateTime      *datetime,
-                                                                         const gchar    *format);
+                                                                         const gchar    *format) G_GNUC_MALLOC;
 
 G_END_DECLS
 

glib/tests/gdatetime.c

@@ -25,14 +25,6 @@
 #include <glib/gstdio.h>
 #include <locale.h>
 
-/* The tests below exercise invalid formats and y2k-unsafe formats,
- * so we need to silence the warnings here to pass.
- */
-#ifdef __GNUC__
-#pragma GCC diagnostic ignored "-Wformat"
-#pragma GCC diagnostic ignored "-Wformat-y2k"
-#endif
-
 #define ASSERT_DATE(dt,y,m,d) G_STMT_START { \
   g_assert_nonnull ((dt)); \
   g_assert_cmpint ((y), ==, g_date_time_get_year ((dt))); \

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[])
 {