flatpak: Drop GSK_RENDERER env var

We want to use the default renderer.

This env var was introduced in
commit df4c57c001 to work around an
inspector bug.

But it's not 2020 anymore and Vulkan actually works now.

build-aux/flatpak/org.gtk.Demo4.json

@@ -197,9 +197,4 @@
             ]
         }
     ],
-    "build-options" : {
-        "env" : {
-            "GSK_RENDERER" : "opengl"
-        }
-    }
 }

build-aux/flatpak/org.gtk.WidgetFactory4.json

@@ -129,7 +129,6 @@
     "build-options" : {
         "env" : {
             "DBUS_SESSION_BUS_ADDRESS" : "''",
-            "GSK_RENDERER" : "opengl",
             "G_ENABLE_DEBUG" : "true"
         }
     }

build-aux/flatpak/org.gtk.gtk4.NodeEditor.json

@@ -129,7 +129,6 @@
     "build-options" : {
         "env" : {
             "DBUS_SESSION_BUS_ADDRESS" : "''",
-            "GSK_RENDERER" : "opengl",
             "G_ENABLE_DEBUG" : "true"
         }
     }