Remove deprecated experimental invariant_booleans lint rule (#91)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 7b7d7ce..51df836 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -15,7 +15,6 @@
     - cascade_invocations
     - comment_references
     - directives_ordering
-    - invariant_booleans
     - join_return_with_assignment
     - lines_longer_than_80_chars
     - missing_whitespace_between_adjacent_strings