vt_vga: When clearing video memory, don't read from it

The goal is to clear the video memory, in case an application drew to
it. So the content shouldn't be loaded in the latches, it can't be
trusted anyway.

This improves a bit the window switch speed.

MFC after:	1 week

sys/dev/vt/hw/vga/vt_vga.c

@@ -604,7 +604,6 @@ vga_initialize(struct vt_device *vd, int textmode)
 		 * planes.
 		 */
 		for (ofs = 0; ofs < VT_VGA_MEMSIZE; ofs++) {
-			MEM_READ1(sc, ofs);
 			MEM_WRITE1(sc, ofs, 0);
 		}
 	}