adb: remove atransport ref counting.

The reference count is always 0 or 1.

Test: ./test_adb.py
Change-Id: I669f98f4996f4e41ac037f1add9c47819d4003d4
diff --git a/adb/transport.h b/adb/transport.h
index 89d76b8..0de3859 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -266,7 +266,7 @@
     usb_handle* GetUsbHandle() { return usb_handle_; }
 
     const TransportId id;
-    size_t ref_count = 0;
+
     bool online = false;
     TransportType type = kTransportAny;