remove no_implicit_optional

this is the default in mypy 0.990

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

setup.cfg

diff --git a/setup.cfg b/setup.cfg
index d4658e2..30688d9 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -75,7 +75,6 @@
 disallow_any_generics = true
 disallow_incomplete_defs = true
 disallow_untyped_defs = true
-no_implicit_optional = true
 warn_redundant_casts = true
 warn_unused_ignores = true