input: synaptics: remove outdated header

Remove the inclusion of the outdated (and now unused) asm/system.h.

Change-Id: Ic2081c61f4596d74560e00ffb4d486f7519ef473
Acked-by: Christian Bolis <cbolis@qti.qualcomm.com>
Signed-off-by: Keith Fallows <keithf@codeaurora.org>