dm: fix DM_EVENT dependencies

CONFIG_DM_EVENT without CONFIG_EVENT is non-functional.
Let CONFIG_DM_EVENT depend on CONFIG_EVENT.

Remove superfluous stub in include/event.h.

Fixes: 5b896ed5856f ("event: Add events for device probe/remove")
Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/drivers/core/Kconfig b/drivers/core/Kconfig
index 27d6578..9b9a714 100644
--- a/drivers/core/Kconfig
+++ b/drivers/core/Kconfig
@@ -89,8 +89,7 @@
 
 config DM_EVENT
 	bool "Support events with driver model"
-	depends on DM
-	imply EVENT
+	depends on DM && EVENT
 	default y if SANDBOX
 	help
 	  This enables support for generating events related to driver model