Don't add an asignee in the package issue template

[dstansby]

The motivation behind this is:

1. In my experience it's rare for a single person to be assigned every issue in a repository. Much more common are no-one is assigned by default, multiple people (e.g. core developers) are assigned by default, or a group is assigned by default.
2. Not having a GitHub user in the issue template makes it much easier to fix github_user does not indicate value can be an organisation #305 by changing the github_user field to stand for either a user or organisation.

[paddyroddy]

Are we sure it will work when assignees isn't present altogether? Should we be doing

assignees:

[dstansby]

I've rebase on to the schema validation PR, and removing assignee altogether passes the validation.

[dstansby]

Whoever merges please squash merge!