auxdisplay: hd44780: Remove clear_fast

We remove the hd44780_clear_fast (display) clear implementation. With
the new timeout the normal clear_display is reasonably fast. So there is
no need for a clear_fast anymore.

Link: https://lore.kernel.org/lkml/20200922092121.GG16421@1wt.eu/

Signed-off-by: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>