Enable lint unnecessary_statements. Codebase is compliant. (#698)

analysis_options.yaml

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 8626116..fa320d8 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -127,6 +127,7 @@
     - unnecessary_null_aware_assignments
     - unnecessary_null_in_if_null_operators
     # - unnecessary_overrides # https://github.com/dart-lang/linter/issues/626 and https://github.com/dart-lang/linter/issues/627
+    - unnecessary_statements
     - unnecessary_this
     - use_rethrow_when_possible
     # - use_setters_to_change_properties # not yet tested