Remove `/python sourcepath`

I feel like this mostly is confusing people.
Also don't see much value for it.

To me it looks like a regular workflow is like:
```
/plugin install ~/src/profanity-plugins/my.py
```

The whole thing with sourcepath, install (which also loads without
having that described anywhere), load etc is confusing.
Also each plugin file that is present in
`.local/share/profanity/plugins` will then be auto loaded. Which means
after installation.

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