Prepare to drop some CharmStore methods.

[ericsnowcurrently]

The CharmStore type has several exported methods that are not part of Interface. If the goal is to have CharmStore be a strictly conforming implementation then these methods do not belong. With the addition of CharmStore.Client() we can remove all but one of the methods. They are either unnecessary or may be implemented as needed in juju core. The one remaining method is Latest(). This patch moves it to csclient.Client.

Once core has been updated appropriately, the entire charmstore_going_away.go file may be removed.

[jujugui]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
http://ci.jujugui.org:8080//job/charmrepo/216/
Test PASSed.

[ericsnowcurrently]

Per PR #75, the directive relative to CharmStore is pretty clear.

[natefinch]

LGTM

[ericsnowcurrently]

[jujugui]

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