Fix SEGV crash by unref css provider too much

Fix SEGV crash by unref css provider too much

I Got css provider from F, and was going to remove it from style
context.  But between them, unhighlight was called when
gtk_container_remove was called, and the css provider was already
freed, so I can't use the css provider to remove it from style
context.

As a fix, I call gtk_container_remove after remove it.
Also, I free a css provider set by unhighlight.

* src/pgtkterm.c (x_set_parent_frame):