Remove the "Edit Multiple Files" external editor setting

Summary: Ref T13515. No callsites actually use this, most editors don't support it, it doesn't seem terribly useful for the ones that do, it makes template-based APIs for line-number substitution complicated, and we can probably just loop on window.open() anyway.

Test Plan: Grepped for affected symbols, found no more references. Loaded settings page, saw no more setting.

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21142