Removing logback config from the library JAR

Since there's a `logback.xml` in the Plaid library JAR, it conflicts with the `logback.xml` I'm using to configure my full application.  It's generally considered a bad practice to include a `logback.xml` in your JAR (see http://stackoverflow.com/a/18275945/17697 for a discussion of the subject), so this PR proposes to just delete the file.  If users want some recommended logback config, they can reference [the test logback config](https://github.com/plaid/plaid-java/blob/master/src/test/resources/logback.xml).