Remove GLX support

EGL is the de-facto GL context initialization API, including on X11
where it provides many additional features over GLX.

I’m planning on adding support for selecting the GPU (adapter in
Dolphin-speak) also to OpenGL, similarly to the Vulkan backend, and that
will require EGL, so let’s remove the legacy API first.

Source/Core/Common/GL/GLContext.h

@@ -47,8 +47,7 @@ public:
   // Creates an instance of GLContext specific to the platform we are running on.
   // If successful, the context is made current on the calling thread.
   static std::unique_ptr<GLContext> Create(const WindowSystemInfo& wsi, bool stereo = false,
-                                           bool core = true, bool prefer_egl = false,
-                                           bool prefer_gles = false);
+                                           bool core = true, bool prefer_gles = false);
 
 protected:
   virtual bool Initialize(const WindowSystemInfo& wsi, bool stereo, bool core);