Remove iso bootloader wallpaper and default resolution.

It doesn't scale well to large resolutions or other aspect ratios. This
fully lets the bootloader control the graphical resolution and will likely
load faster.

isosrc/boot/grub/grub.cfg

@@ -1,23 +1,13 @@
 insmod part_msdos
 insmod ext2
-insmod png
 
 if loadfont /boot/grub/fonts/unicode.pf2 ; then
-  set gfxmode=800x600
   insmod vbe
   insmod vga
   insmod gfxterm
 fi
 terminal_output gfxterm
 
-if background_image /boot/grub/sortix.png ; then
-  set color_normal=white/black
-  set color_highlight=white/light-gray
-else
-  set menu_color_normal=white/black
-  set menu_color_highlight=white/light-gray
-fi
-
 set timeout=10
 set default="0"