rubocop/config/default: Unset DisabledByDefault

- The `DisabledByDefault` config option made it so that "all cops in the
  default configuration are disabled, and only cops in user
  configuration are enabled", which makes cops "opt-in rather than
  opt-out" (source:
  https://github.com/rubocop/rubocop/blob/d1270c468cdb9a211229e36aef8d568ae3bfe607/config/default.yml#L91-L102).
- It's not immediately obvious that this gem has this config option.
  This means that people may write new, custom cops in downstream
  projects, with no configuration in .rubocop.yml, and then they never run
  because all cops are disabled unless explicitly enabled or otherwise
  configured with `Include`/`Exclude`.
- RuboCop does not warn users that the config inheritance has set
  `DisabledByDefault` somewhere up the line, leading to users mistakenly
  not enabling cops they may have intended to.
- Other of GitHub's open source gems, like `rubocop-github`[1] and
  `rubocop-rails-accessibility`[2], have moved away from
  `DisabledByDefault`. This is the last one (at least that I can find in
  GitHub's main project's RuboCop gem inheritance tree).

[1] - github/rubocop-github#119
[2] - github/rubocop-rails-accessibility#7

lib/rubocop/config/default.yml

@@ -1,9 +1,6 @@
 require:
   - rubocop/cop/primer
 
-AllCops:
-  DisabledByDefault: true
-
 Primer/SystemArgumentInsteadOfClass:
   Enabled: true