Drop FGET and revert read access to Lisp_Objects slots of struct frame.

Drop FGET and revert read access to Lisp_Objects slots of struct frame.
* frame.h (FGET): Remove.
(struct frame): Do not use INTERNAL_FIELD.
* buffer.c, data.c, dispnew.c, dosfns.c, eval.c, fontset.c, frame.c:
* fringe.c, gtkutil.c, minibuf.c, msdos.c, nsfns.m, nsmenu.m, nsterm.m:
* print.c, term.c, w32fns.c, w32menu.c, w32term.c, window.c, window.h:
* xdisp.c, xfaces.c, xfns.c, xmenu.c, xterm.c: Adjust users.