Changes to the plugin.py file to include the plugin remove_jar feature

[nanaessuman]

I added "remove_jar" to plugin.jar to allow users to remove jars from directories when they add files with the add_jar feature.

[rschlussel-zz]

Thanks for the patch! We're excited to have a new contributor! A couple notes on testing: You should run 'make lint' to ensure that the code adheres to our style. It's best to test with at least one presto node on a different node from presto-admin. Usually we write product tests to be able to run this automatically, but it's a bit complicated to set up, so it's fine if you just verify manually.

[kokosing]

@nanaessuman Could you please address the comments from @rschlussel ? So we could merge this PR.