video: fbdev: efifb: Fix set but not used warning for screen_pitch

screen_pitch was asssigned a value which was never used.
Drop it to fix the warning

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Peter Jones <pjones@redhat.com>
Cc: linux-fbdev@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20201206190247.1861316-12-sam@ravnborg.org