Drop deprecated mypy option

show_error_codes is deprecated, and represents the default now.

Author: EliahKagan

pyproject.toml

@@ -26,7 +26,6 @@ no_implicit_optional = true
 warn_redundant_casts = true
 warn_unused_ignores = true
 warn_unreachable = true
-show_error_codes = true
 implicit_reexport = true
 # strict = true