fold preparing inferior into starting inferior

after refactoring the trk adapter startup, the split was not necessary
any more. in fact, it seemed fairly arbitrary at this point.

src/plugins/debugger/gdb/coregdbadapter.h

@@ -55,7 +55,6 @@ public:
     bool dumpersAvailable() const { return false; }
 
     void startAdapter();
-    void prepareInferior();
     void startInferior();
     void interruptInferior();
     void shutdown();