Fix issue with use mro 'c3' in plugins

This PR attempts to fix an issue with the mro pragma. Under mro 'c3', the code

```perl
use Dancer::Plugin;
```

is problematic as it results in an exception: "Inconsistent hierarchy during C3 merge of class '...'". The issue is simply that plugins, after registration, have an @ISA consisting of Exporter, Dancer::Plugin, and their own @ISA, but Dancer::Plugin already inherits from Exporter and so the inheritance hierarchy is not C3-friendly.

Simply removing Exporter from the list of classes added to the plugin's @ISA seems to fix the issue.