adapter: Don't use DBG in mgmt_debug

mgmt_debug callback is used to print debug strings from mgmt instances
which includes the file and function names so using DBG would add yet
another set of file and function prefixes which makes the logs
confusing.

src/adapter.c

@@ -10327,9 +10327,7 @@ static void read_version_complete(uint8_t status, uint16_t length,
 
 static void mgmt_debug(const char *str, void *user_data)
 {
-	const char *prefix = user_data;
-
-	info("%s%s", prefix, str);
+	DBG_IDX(0xffff, "%s", str);
 }
 
 int adapter_init(void)
@@ -10342,8 +10340,7 @@ int adapter_init(void)
 		return -EIO;
 	}
 
-	if (getenv("MGMT_DEBUG"))
-		mgmt_set_debug(mgmt_primary, mgmt_debug, "mgmt: ", NULL);
+	mgmt_set_debug(mgmt_primary, mgmt_debug, NULL, NULL);
 
 	DBG("sending read version command");