Remove deprecated experimental invariant_booleans lint rule (#76)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 45de1ba..e549d4d 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -15,7 +15,6 @@
     - cancel_subscriptions
     - comment_references
     - directives_ordering
-    - invariant_booleans
     - literal_only_boolean_expressions
     - no_adjacent_strings_in_list
     - omit_local_variable_types