Remove deprecated experimental invariant_booleans lint rule (#42)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 56bbaad..200a3d4 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -26,7 +26,6 @@
     - empty_statements
     - hash_and_equals
     - implementation_imports
-    - invariant_booleans
     - iterable_contains_unrelated_type
     - library_names
     - library_prefixes