Revert "ephy-embed-single: also initialize nspluginwrapper plugins in WebKit2"

The only plugin nspluginwrapper could really handle was Flash. Flash has
had a 64-bit version for a while, and isn't available separately from
Google Chrome any more. So, nspluginwrapper doesn't have anything to
support, ergo, let's remove nspluginwrapper support.

This reverts commit 09f082567a909bc9369f57a2e14b28846057814e.

https://bugzilla.gnome.org/show_bug.cgi?id=720758