Input: surface3_spi - remove set but unused variable 'timestamp'

Input: surface3_spi - remove set but unused variable 'timestamp'

Fixes the following W=1 kernel build warning(s):

 drivers/input/touchscreen/surface3_spi.c: In function ‘surface3_spi_process_touch’:
 drivers/input/touchscreen/surface3_spi.c:97:6: warning: variable ‘timestamp’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Link: https://lore.kernel.org/r/20210114152323.2382283-5-lee.jones@linaro.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>