Revert "Platform: choose a GLXFBConfig with a pbuffer."

Caused an error when creating a WindowlessGlxApplication on a Mesa with
just llvmpipe. Note to self: never again try to fix anything related to
Xlib that's not broken.

This reverts commit 511d0c1b27.

src/Magnum/Platform/WindowlessGlxApplication.cpp

@@ -85,13 +85,9 @@ WindowlessGlxContext::WindowlessGlxContext(const WindowlessGlxContext::Configura
         return;
     }
 
-    /* Choose a config that supports a pbuffer (by default it's GLX_WINDOW_BIT
+    /* Choose config */
-       which we don't need at all) */
     int configCount = 0;
-    constexpr static const int fbAttributes[] = {
+    constexpr static const int fbAttributes[] = { None };
-        GLX_DRAWABLE_TYPE, GLX_PBUFFER_BIT,
-        None
-    };
     GLXFBConfig* configs = glXChooseFBConfig(_display, DefaultScreen(_display), fbAttributes, &configCount);
     if(!configCount) {
         Error() << "Platform::WindowlessGlxContext: no supported framebuffer configuration found";