Remove "pretag" hook support in Mercurial

Summary: Ref T4195. This doesn't actually work like I thought it did: it only fires locally, when you run `hg tag`. Mercurial tags are also weird and basically don't make any sense and everyone should use bookmarks instead. We could implement some flavor of this eventually, but I'd like to see users request it first. They can implement their own with content-based hooks once those work, anyway.

Test Plan: This code didn't do anything.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7765