Made unregister remove passphrase and suite.rc.processed.

A small improvement: ```cylc unregister``` removes ```passphrase``` and ```suite.rc.processed``` from the local suite directory, if they exist. 

The passphrase is only needed by registered suites, and is auto-generated on registering, so I figure that's a no-brainer.  That the processed suite definition should be auto-removed is less obvious (I could be dissuaded) ... but it means we could temporarily register a suite, e.g. to test something, and then unregister it, without leaving any crud behind in the directory.

@matthewrmshin - please consider, and review.