rubocop: Intentionally disable RSpec/{DescribeClass,StubbedMock}

- These had a lot of offenses that were marked as "try to enable".
- A lot of the "describe class" ones were for tests for cmds or dev-cmds,
  `brew typecheck` or `brew --env`, and the cop would only pass if I
  changed these "describe"s to `BrewTypecheck` or `Brew__Env` which
  seemed unhelpful.
- The usefulness of the stubbed mocks cop is
  [disputed](https://github.com/rubocop/rubocop-rspec/issues/ 1271), and
  fixing the offenses (not autocorrectable) would involve us doing a
  fair number of changes since changing `expect` to `allow` would then
  force us to make each of the affected tests have `expect` assertions
  at the end, where they sometimes don't currently.

Library/.rubocop.yml

@@ -274,8 +274,12 @@ Rails/ToFormattedS:
 # Intentionally disabled as it doesn't fit with our code style.
 RSpec/AnyInstance:
   Enabled: false
+RSpec/DescribeClass:
+  Enabled: false
 RSpec/FilePath:
   Enabled: false
+RSpec/StubbedMock:
+  Enabled: false
 RSpec/SubjectStub:
   Enabled: false
 
@@ -283,12 +287,6 @@ RSpec/SubjectStub:
 RSpec/MessageSpies:
   EnforcedStyle: receive
 
-# TODO: try to enable these
-RSpec/DescribeClass:
-  Enabled: false
-RSpec/StubbedMock:
-  Enabled: false
-
 # These were ever-growing numbers, not useful.
 RSpec/ExampleLength:
   Enabled: false