Remove dead Gtk[Rc]Style code

Remove dead Gtk[Rc]Style code

Most code in gtkrc.c has been turned into a no-op, but that one
reverting in public API (gtk_rc_scanner_new() and such). GtStyle
is also more shallow, now fully relies in the backing
GtkStyleContext and all connection to gtkrc.c has been removed.

GtkBinding has been also affected, there is no replacement yet
for custom keybindings in style files, so that piece of code that
hooked into gtkrc has been replaced by a FIXME so in the future
it may be added back.