adb: remove meaningless const.

Returning a const value is pointless.

Test: mma
Change-Id: Ia578d75345361d53e4a29bb9724bfcc161b8d8be
diff --git a/adb/transport.h b/adb/transport.h
index a492f00..8b71e34 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -204,8 +204,8 @@
     char token[TOKEN_SIZE] = {};
     size_t failed_auth_attempts = 0;
 
-    const std::string serial_name() const { return serial ? serial : "<unknown>"; }
-    const std::string connection_state_name() const;
+    std::string serial_name() const { return serial ? serial : "<unknown>"; }
+    std::string connection_state_name() const;
 
     void update_version(int version, size_t payload);
     int get_protocol_version() const;