Clean up more code from PGTK port

Clean up more code from PGTK port

* lisp/term/pgtk-win.el (pgtk-disown-selection-internal)
(pgtk-get-selection-internal): Fix declarations for new calling
conventions.
* src/gtkutil.c (xg_frame_set_char_size, x_wm_set_size_hint):
Clean up meaningless variables on PGTK.
* src/pgtkfns.c (Fx_create_frame, syms_of_pgtkfns): Clean up
meaningless variables and fix copied doc strings to use "skip".
* src/pgtkim.c (im_context_commit_cb)
(im_context_retrieve_surrounding_cb)
(im_context_delete_surrounding_cb, make_color_string)
(im_context_preedit_changed_cb, im_context_preedit_end_cb)
(im_context_preedit_start_cb): Fix coding style.
* src/pgtkselect.c (pgtk_selection_usable)
(Fpgtk_disown_selection_internal, Fpgtk_get_selection_internal):
Remove unused arguments.
(syms_of_pgtkselect): Fix doc strings and old style variable
declarations.
* src/pgtkterm.c (x_set_offset, x_set_parent_frame)
(syms_of_pgtkterm): Clean up doc strings and remove meaningless
variables.