Update for removal of WebKitGTK sandbox API

Merged Will Thompson requested to merge wip/wjt/webkitgtk-sandbox-api-removal-43 into gnome-43

WebKitGTK 2.39.5 contains an ABI break. The sandbox is now mandatory, so the API to enable/disable it is gone.

(cherry picked from commit 1a8f9ecf)

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