[analyzer/linter] Remove unused lint rule description field

Looks like this was added in https://dart-review.googlesource.com/c/sdk/+/255280, but since the longer descriptions are stored in and used from the messages.yaml file, it looks like it's not needed.

Change-Id: I2feb26eeda7c3d76f2567d9e0a894acd25d713ff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/384825
Commit-Queue: Brian Wilkerson <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: parlough

pkg/analyzer/lib/src/lint/linter.dart

@@ -206,11 +206,6 @@ abstract class LintRule {
   /// Lint name.
   final String name;
 
-  /// The documentation for the lint that should appear on the Diagnostic
-  /// messages page. This field should never be accessed in any code in `lib` or
-  /// `bin`.
-  final String? documentation;
-
   /// The state of a lint, and optionally since when the state began.
   final State state;
 
@@ -220,7 +215,6 @@ abstract class LintRule {
     required this.description,
     required this.details,
     State? state,
-    this.documentation,
   }) : state = state ?? State.stable();
 
   /// Indicates whether the lint rule can work with just the parsed information