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/geometry_pipeline.cpp

@@ -255,7 +255,7 @@ public:
     }
 
 private:
-    const Regs& regs;
+    [[maybe_unused]] const Regs& regs;
     Shader::ShaderSetup& setup;
     Common::Vec4<float24>* buffer_begin;
     Common::Vec4<float24>* buffer_cur;