remove no_implicit_optional

this is the default in mypy 0.990

Committed via https://github.com/asottile/all-repos

Author: asottile

setup.cfg

@@ -96,7 +96,6 @@ mypy_path = src
 check_untyped_defs = True
 disallow_any_generics = True
 ignore_missing_imports = True
-no_implicit_optional = True
 show_error_codes = True
 strict_equality = True
 warn_redundant_casts = True