ARM: OMAP2+: remove unused omap4-keypad file and code

This has been unused since omap4 board files went away.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>