Remove uses of charm.URL.Channel

[babbageclunk]

This has been removed from charm.URL in the process of implementing http://pad.lv/1584193.

The only uses were checking that the url had no channel or removing the channel from a url.

Updated charm.v6-unstable dependency. This is arguably unnecessary, but it's safer - otherwise someone could pass in a charm url with a channel and this code would no longer strip it out.

[jujugui]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
http://ci.jujugui.org:8080//job/charmstore/1902/
Test FAILed.

[jujugui]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
http://ci.jujugui.org:8080//job/charmstore/1903/
Test PASSed.

[frankban]

Looks good, thanks for this!

[babbageclunk]

[jujugui]

Status: merge request accepted. Url: http://ci.jujugui.org:8080/job/charmstore-merge