configs, LICENSE: remove pbx.digium.com.

pbx.digium.com no longer accepts IAX2 calls and
there are no plans for it to come back.

Accordingly, nonworking IAX2 URIs are removed from
both the LICENSE file and the sample config.

ASTERISK-29923 #close

Change-Id: I257c54d4d812ed6b4bd4cbec2cd7ebe2b87b5bad