input: remove now deprecated corgi_ts.c touchscreen driver

The corgi touchscreen is now deprecated in favour of the generic ads7846.c
driver. The noise reduction technique used in corgi_ts.c, which is to wait
till vsync before ADC sampling, is also integrated into ads7846 driver now.
Provided that the original driver is not generic and is difficult to maintain,
it will be removed now.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>