Purge jar-specific handling that is not needed anymore.

This removes all monkeypatches for JRUBY-6970, woohoo!
I also tried to find any references to jar, jar:, or file: and
remove the special handling there.

`make test` says it's cool, so I'm happy enough.

    Finished in 19.7 seconds
    502 examples, 0 failures
Refacter known file lookups to a single method in LogStash::Environment
Merge pull request #1325 from jordansissel/purge-JRUBY-6970

Purge jar-specific handling that is not needed anymore.