adb: Cleanup dead code

dns_service is unused and recover_service has been replaced by adb
sideload

Change-Id: Ie90000d7f672e8299ee1622a9690c7371b214dc1
diff --git a/adb/adb.h b/adb/adb.h
index a01d460..38767e5 100644
--- a/adb/adb.h
+++ b/adb/adb.h
@@ -128,10 +128,7 @@
         */
     void (*close)(asocket *s);
 
-        /* socket-type-specific extradata */
-    void *extra;
-
-    	/* A socket is bound to atransport */
+        /* A socket is bound to atransport */
     atransport *transport;
 };