Remove scss/at-mixin-pattern rule

[bjankord]

Resolves #181
Resolves #191

Update to allow people to write mixins however they need.
If you find that you need this functionality, you can add the rule back to your config with the following:

{
  "extends": "stylelint-config-sass-guidelines",
  "rules": {
    "scss/at-mixin-pattern": "^[a-z]+([a-z0-9-]+[a-z0-9]+)?$",
  }
}