main/gtk+2.0: fix commands in .post-install
The files it wrote too haven't been used by GTK for quite a long time since they have been moved to /usr. Instead the --update-cache parameter can be used to update those files, and there is no need to specify a location anymore.
Fixes #13170 (closed)
Follow-up from: 90fcd62c
Follow-up from: 1df1445c