Remove default Magespec config

Now that develop is tracking phpspec master, we can benefit from a bug fix that allows custom config files to be passed to phpspec as a property of the Application Input object. This allows us to set the config values required to run Magespec during Behat execution, meaning that there is no need to have the standard Magespec configuration in the project root.

Removing the config prevents https://github.com/MageTest/MageSpec/issues/74 from affecting development on the extension itself.