vgabios: Remove special case of dh==0xff in handle_1013()

vgabios: Remove special case of dh==0xff in handle_1013()

The original "lgpl vgabios" had a special case for dh==0xff in its
int1013 (write string) code.  There does not appear to be any VGABIOS
documentation supporting this as an externally available feature.  It
appears this was for its own internal use when writing its strings to
the screen.  SeaVGABIOS doesn't use this hack; this patch removes it
from the code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>