Delete unused callback on isSlugUnique function

[lindapaiste]

Ref #521
This is part of a larger effort to move away from callback-based code.

Quick & easy cleanup. projectSchema.methods.isSlugUnique supports an optional callback cb argument. But the one place in the code where we use this function is already using it as an async. So we can safely delete all logic related to the callback.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[raclim]

Thanks for catching this, it's so much easier to read!