Input: synaptics-rmi4 - remove set but not used variable 'sensor_flags'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/input/rmi4/rmi_f12.c: In function rmi_f12_read_sensor_tuning:
drivers/input/rmi4/rmi_f12.c:76:6: warning: variable sensor_flags set but not used [-Wunused-but-set-variable]

It's not used since introduction in
commit b43d2c1e ("Input: synaptics-rmi4 - add support for F12")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>