Revert "use new cmd command"

This reverts commit d039800a4e008b52c532f901e3bb34b40877cf30.

Change-Id: Ia9b81c47120088593a56ae09a9b002338d524b8a
diff --git a/adb/transport.h b/adb/transport.h
index d9845b6..f41a8d4 100644
--- a/adb/transport.h
+++ b/adb/transport.h
@@ -38,9 +38,7 @@
 
 // Do not use any of [:;=,] in feature strings, they have special meaning
 // in the connection banner.
-extern const char* const kFeatureShell2;
-// The 'cmd' command is available
-extern const char* const kFeatureCmd;
+constexpr char kFeatureShell2[] = "shell_v2";
 
 class atransport {
 public: