Delete variable window-system-version obsolete since 24.3

Delete variable window-system-version obsolete since 24.3

* lisp/frame.el (window-system-version): Delete variable.
* src/dispnew.c (syms_of_display) <Vwindow_system_version>:
Delete DEFVAR, obsolete since 24.3.
(init_display_interactive, syms_of_display_for_pdumper):
* src/msdos.c (internal_terminal_init):
* src/nsterm.m (ns_term_init):
* src/pgtkfns.c (pgtk_display_info_for_name):
* src/w32fns.c (w32_display_info_for_name, Fx_open_connection):
* src/xfns.c (x_display_info_for_name, Fx_open_connection): Don't
set above deleted variable.
* admin/admin.el (set-version): Don't update above deleted
variable in msdos.c.
* lisp/textmodes/artist.el (artist-submit-bug-report): Don't use
above deleted variable.