Remove support for browser plugins

All browser vendors agree: plugins were a mistake. I no longer intend to
support browser plugins in Epiphany. Windowed plugins only work in X11,
the amount of crash reports from plugins is absurd, it's time for the
GNOME Shell browser plugin to go, Evince browser plugin is far too
buggy, the only plugin that still really matters is Flash, and Firefox
and Chrome are slowly killing that too. If you want Flash, go use one
of those browsers or MiniBrowser or whatever.

Note that plugins are unrelated to extensions. We do wish to support
Firefox/Chrome WebExtensions. Everyone likes extensions.