core: Bugfix, use correct config parameter string.

I submitted a pull request to Richard as well:
https://bitbucket.org/richard42/mupen64plus-core/pull-request/61/bugfix-use-correct-config-parameter-string/diff
front: Remove unneeded persistence of slot number.

The core writes this directly to the native config file, so we don't
need to bother setting, tracking, or persisting it.