Remove usage of the GDK_SCALE variable

Remove usage of the GDK_SCALE variable

* src/gtkutil.c (xg_get_gdk_scale): Remove.
(xg_get_default_scrollbar_height)
(xg_get_default_scrollbar_width): Pass in a frame to check for
scaling.
(xg_frame_set_char_size): Use the API for querying scale
instead of looking at the GDK_SCALE variable.
(xg_get_default_scrollbar_width): Ditto.
(xg_get_default_scrollbar_height): Ditto.
(xg_update_scrollbar_pos): Ditto.

* src/xfns.c (x_set_scroll_bar_default_height): Pass in the
frame to get the width.