Remove deprecated lint "prefer_bool_in_asserts". (#24890)

In Dart 2, asserts no longer accept non-bool values so this rule is
made redundant by the Dart analyzer's basic checks and is no longer
necessary.

The rule will be removed in a future Linter release.

analysis_options.yaml

@@ -114,7 +114,6 @@ linter:
     # - parameter_assignments # we do this commonly
     - prefer_adjacent_string_concatenation
     - prefer_asserts_in_initializer_lists
-    - prefer_bool_in_asserts
     - prefer_collection_literals
     - prefer_conditional_assignment
     - prefer_const_constructors