Debugger: Combine some gdb startup code paths

After splitting out terminal and core handling they are
essentially the same.

Change-Id: I603cea74186eada5728295b430c9385812dcc4cd
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/debugger/gdb/gdbengine.h

@@ -96,7 +96,6 @@ private: ////////// General Interface //////////
 
     ////////// Gdb Process Management //////////
 
-    void startGdb(const QStringList &args = QStringList());
     void handleInferiorShutdown(const DebuggerResponse &response);
     void handleGdbExit(const DebuggerResponse &response);
     void setLinuxOsAbi();