pm: device: remove redundant initialization field

Z_PM_DEVICE_INIT is used as a static initializer, so all non-specified
fields will already be set to zero.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
diff --git a/include/pm/device.h b/include/pm/device.h
index 5c72025..fef1968 100644
--- a/include/pm/device.h
+++ b/include/pm/device.h
@@ -117,7 +117,6 @@
 
 #ifdef CONFIG_PM_DEVICE_RUNTIME
 #define INIT_PM_DEVICE_RUNTIME(obj)			\
-	.usage = 0U,					\
 	.lock = Z_MUTEX_INITIALIZER(obj.lock),		\
 	.condvar = Z_CONDVAR_INITIALIZER(obj.condvar),
 #else