Remove superfluous error (dart-lang/source_gen#144)

This is already an error – no points for duplication

Author: kevmoo

pkgs/source_gen/analysis_options.yaml

@@ -3,7 +3,6 @@ analyzer:
  errors:
    unused_import: error
    unused_local_variable: error
-   invalid_assignment: error
    dead_code: error
 linter:
   rules: