[linter] Introduce a new representation for removed lints.

Previously, the process for removing a lint from `pkg/linter` was the
following:

- Remove the lint class's override of
  `AbstractAnalysisRule.registerNodeProcessors` (effectively changing
  the rule into a no-op).

- Change its override of `AnalysisRule.diagnosticCode` to return the
  pseudo-diagnostic code `removedLint`.

- Modify its constructor to use `RuleState.removed`, so that the lint
  would be marked as being in the "removed" state.

This change introduces a new class, `RemovedAnalysisRule`, as the
standard way to represent an analysis rule that has been removed. So
the new process for removing a lint from `pkg/linter` will be to
remove its class entirely and instead register an instance of
`RemovedAnalysisRule`.

This avoids the need for the pseudo-diagnostic code `removedLint` to
exist at all, and also makes the representation of a removed lint much
more compact.

To help encourage clients to use the new `RemovedAnalysisRule` class,
the `RuleState.removed` constructor has been deprecated. It will be
removed in a future version of the analyzer.

Change-Id: I6a6a6964726595b7bb32664846cf4e4722bbb4f1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/463463
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>