pm: device: remove unused flag

PM_DEVICE_FLAG_COUNT is not used, so remove it.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/include/pm/device.h b/include/pm/device.h
index c37a16d..9227be8 100644
--- a/include/pm/device.h
+++ b/include/pm/device.h
@@ -56,8 +56,6 @@
 	PM_DEVICE_FLAGS_WS_CAPABLE,
 	/** Indicates if the device is being used as wakeup source. */
 	PM_DEVICE_FLAGS_WS_ENABLED,
-	/** Number of flags (internal use only). */
-	PM_DEVICE_FLAG_COUNT
 };
 
 /** @brief Device PM actions. */