gl_rasterizer_cache: Remove unused FlushSurface method.

src/video_core/renderer_opengl/gl_rasterizer_cache.cpp

@@ -1121,10 +1121,6 @@ void RasterizerCacheOpenGL::LoadSurface(const Surface& surface) {
     surface->UploadGLTexture(read_framebuffer.handle, draw_framebuffer.handle);
 }
 
-void RasterizerCacheOpenGL::FlushSurface(const Surface& surface) {
-    surface->FlushGLBuffer();
-}
-
 Surface RasterizerCacheOpenGL::GetSurface(const SurfaceParams& params, bool preserve_contents) {
     if (params.addr == 0 || params.height * params.width == 0) {
         return {};

src/video_core/renderer_opengl/gl_rasterizer_cache.h

@@ -867,9 +867,6 @@ public:
     /// Get the color surface based on the framebuffer configuration and the specified render target
     Surface GetColorBufferSurface(std::size_t index, bool preserve_contents);
 
-    /// Flushes the surface to Switch memory
-    void FlushSurface(const Surface& surface);
-
     /// Tries to find a framebuffer using on the provided CPU address
     Surface TryFindFramebufferSurface(VAddr addr) const;