NSX: drop support to deprecated dist-router extension

The NSX plugin originally implemented its own 'dist-router' extension.
During the Juno timeframe the DVR extension was introduced and the NSX
plugin was ported to support both. At the same time 'dist-router' was
marked for removal in Kilo.

Now that Kilo opened, we can drop the deprecated one.

Change-Id: I7d6d2524ef6d5cb94a9e8514425a1a288eea7210
Merge "NSX: drop support to deprecated dist-router extension"