staging: fbtft: remove unused bl_ops from fbtft_unregister_backlight

The only usage of bl_ops variable in fbtft_unregister_backlight function
was assigment of a value to that variable, therefore the assignment and
the variable itself can be safely removed

Signed-off-by: Mike Rapoport <mike.rapoport@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>