VideoBackends/Null: Remove unnecessary constructors and destructors

Removes constructors and destructors that don't actually provide any
behavior (i.e. doesn't constain generated code related to non-trivial
members in a cpp file, etc).

Lessens the amount of code present.

Source/Core/VideoBackends/Null/PerfQuery.h

@@ -11,8 +11,6 @@ namespace Null
 class PerfQuery : public PerfQueryBase
 {
 public:
-  PerfQuery() {}
-  ~PerfQuery() override {}
   void EnableQuery(PerfQueryGroup type) override {}
   void DisableQuery(PerfQueryGroup type) override {}
   void ResetQuery() override {}