Remove deprecated experimental invariant_booleans lint rule (#36)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 89fc0e8..d77937e 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -32,7 +32,6 @@
     - file_names
     - hash_and_equals
     - implementation_imports
-    - invariant_booleans
     - iterable_contains_unrelated_type
     - join_return_with_assignment
     - lines_longer_than_80_chars