(#5556): Integrate timer platform details with WatchableEventManager.

[todo]

connectedhomeip/src/include/platform/internal/GenericPlatformManagerImpl_POSIX.cpp

Lines 132 to 136 in c4468cf

	// TODO(#5556): Integrate timer platform details with WatchableEventManager.
	// Let SystemLayer.PrepareSelect() handle timers.
	return CHIP_NO_ERROR;
	}

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This issue was generated by todo based on a TODO comment in c4468cf when #6561 was merged. cc @kpschoedel.