This reverts commits:
 • 9ddcc795
 • ae02adc3

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.