Remove deprecated experimental invariant_booleans lint rule (dart-lang/string_scanner#47)

pkgs/string_scanner/analysis_options.yaml

diff --git a/pkgs/string_scanner/analysis_options.yaml b/pkgs/string_scanner/analysis_options.yaml
index c6528bb..a4495f2 100644
--- a/pkgs/string_scanner/analysis_options.yaml
+++ b/pkgs/string_scanner/analysis_options.yaml
@@ -30,7 +30,6 @@
     - file_names
     - hash_and_equals
     - implementation_imports
-    - invariant_booleans
     - iterable_contains_unrelated_type
     - join_return_with_assignment
     - lines_longer_than_80_chars