QmlDebug: Remove outdated clients

The declarative* clients are only used with Qt Quick 1. We don't need
them anymore.

Change-Id: I102fe93b3acd4b23cc01aff5eb9f12540625fe5e
Reviewed-by: hjk <hjk@qt.io>

src/libs/qmldebug/qmldebugconstants.h

@@ -35,7 +35,5 @@ const char STR_IGNORING_DEBUGGER[] = "Ignoring \"-qmljsdebugger=";
 const char STR_CONNECTION_ESTABLISHED[] = "Connection established";
 const char STR_CONNECTING_TO_SOCKET[] = "Connecting to socket";
 
-const char QDECLARATIVE_ENGINE[] = "QDeclarativeEngine";
-
 } // namespace Constants
 } // namespace QmlDebug