refactor(clock): remove struct waybar_time

The structure was used to pass the locale instance to the date
formatter. All the supported versions of `fmt` are passing the locale
parameter via `FormatContext.locale()` so we can remove the struct and
simplify the code.

While we at it, drop `date::make_zoned` in favor of CTAD on a
`date::zoned_time` constructor.

test/meson.build

@@ -15,7 +15,7 @@ test_src = files(
 
 if tz_dep.found()
   test_dep += tz_dep
-  test_src += files('waybar_time.cpp')
+  test_src += files('date.cpp')
 endif
 
 waybar_test = executable(