giscanner: make _pass_read_annotations2() more readable

_apply_annotations2_function() is only ever called by
_pass_read_annotations2() so there's no need to go and
hide what's being done behind an extra method call.

Also change the 'if not something; return' into a more
readable 'if something: do something'...

https://bugzilla.gnome.org/show_bug.cgi?id=697622