apps: common: event_loop: Remove unused type alias

The type alias `duration` is not used anywhere, so remove it.

Signed-off-by: Barnabás Pőcze <pobrn@protonmail.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>

src/apps/common/event_loop.h

@@ -41,7 +41,6 @@ public:
 	void addFdEvent(int fd, EventType type,
 			std::function<void()> &&handler);
 
-	using duration = std::chrono::steady_clock::duration;
 	void addTimerEvent(const std::chrono::microseconds period,
 			   std::function<void()> &&handler);