code: Cleanup and warning fixes from the Vulkan PR (#6163)

Co-authored-by: emufan4568 <geoster3d@gmail.com>
Co-authored-by: Kyle Kienapfel <Docteh@users.noreply.github.com>

src/video_core/swrasterizer/clipper.cpp

@@ -46,7 +46,7 @@ public:
     }
 
 private:
-    float24 pos;
+    [[maybe_unused]] float24 pos;
     Common::Vec4<float24> coeffs;
     Common::Vec4<float24> bias;
 };