display/pxa2xx_lcd: Remove redundant statement in pxa2xx_palette_parse()

Clang static code analyzer show warning:
hw/display/pxa2xx_lcd.c:596:9: warning: Value stored to 'format' is never read
        format = 0;
        ^        ~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200302130715.29440-8-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>