Tweak deletion of fbstring::operator= taking convertible-to char
Tweak deletion of fbstring::operator= taking convertible-to char

Summary: [Folly] Tweak deletion of `fbstring::operator=` taking convertible-to `char`.

Reviewed By: Orvid

Differential Revision: D13747744

fbshipit-source-id: f254f13a15cb0e72120d1d02f4c8893a788e429a

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly