Drop unnecessary functions that deals with frame pixel size.
* frame.h, msdos.h, w32term.h, xterm.h (x_pixel_width)
(x_pixel_height): Drop prototypes.
* msdos.c, nsfns.m, w32fns.c, xfns.c (x_pixel_width)
(x_pixel_height): Drop implementations.
* frame.c (Fframe_pixel_height): Use FRAME_PIXEL_HEIGHT
which should be always valid for window frame.
(Frame_pixel_width): Likewise with FRAME_PIXEL_WIDTH.
* w32menu.c (Fx_popup_dialog):
* xmenu.c (Fx_popup_dialog): Likewise for both.