Remove the Admin Seeder and Update the CONTRIBUTING.md

[allella]

• Removes a user seeder that was creating a default users that's not needed. It was a bit of a liability to create it because anybody could see the default user info in the public seeder file.
• Add Filament command examples to the CONTRIBUTING.md
• Added a note to CONTRIBUTING.md explaining that, by default, Laravel tests run independent of database seeders, but you can explicitly enable them. There was confusion in Tests: Latest HomeController Tests are Failing #750 (comment) about if seeders were visible to the tests.