adb: remove meaningless const.

Returning a const value is pointless.

Test: mma
Change-Id: Ia578d75345361d53e4a29bb9724bfcc161b8d8be
diff --git a/adb/transport.cpp b/adb/transport.cpp
index 1ccff92..3329f0f 100644
--- a/adb/transport.cpp
+++ b/adb/transport.cpp
@@ -709,7 +709,7 @@
     connection_state_ = state;
 }
 
-const std::string atransport::connection_state_name() const {
+std::string atransport::connection_state_name() const {
     ConnectionState state = GetConnectionState();
     switch (state) {
         case kCsOffline: