[doc] Don’t automatically skip building docs on Windows

Respect the option setting.

.github/workflows/msys2-ci.yml

@@ -61,6 +61,7 @@ jobs:
           meson build \
             --wrap-mode=nodownload \
             --auto-features=enabled \
+            -Ddocs=disabled \
             -Ddirectwrite=enabled \
             -Dgdi=enabled \
             -Dgraphite=enabled \

docs/meson.build

@@ -1,8 +1,3 @@
-if build_machine.system() == 'windows'
-  message('Skipping gtk-doc while building on Windows')
-  subdir_done()
-endif
-
 if not find_program('gtkdoc-scan', required: get_option('docs')).found()
   message('Not building documentation as gtk-doc was not found')
   subdir_done()