Change GPIO port and pin from PA0/1 to PC0/1

Refer to datasheet p.28, PA0 has some problem to be used as GPIO pin at
some case.

New configuration:
PC0: digital input and EXTI0 interrupt.
PC1: digital output, change state periodically.
PG13(onboard LED): toggle while EXTI0 interrupt occurred.

Also remove code of LTDC because it is not related to this experiment
and the pin is conflict.