Convert ledtrig-ide-disk code to use the generic API for one-shot LED
blinking.

This patch changes slightly the behaviour of the trigger, as while the
original version kept the LED on under heavy activity, the new one keeps
a constant on-off blink at 1 / (2 * BLINK_DELAY) Hz.

(bryan.wu@canonical.com: remove 2 useless included header files)
Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>