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