staging: sm750fb: Remove unused function

This patch removes the unused hw712_fillrect function. This patch fixes
the following sparse warning:-

drivers/staging/sm750fb/sm750_accel.c:95:5: warning: symbol 'hw712_fillrect' was not declared. Should it be static?

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>