Remove support for bors commands in review comments

[bryanburgers]

There are 3 types of comments that can be made related to a pull request:

1. Issue comments, which – despite their name – are comments made directly in the pull request. Most of our @bors commands are made here.
2. Review comments, which are comments made on the code diff of a merge request
3. Commit comments, which are comments made on the commit itself (and we do not support in @bors).

I wrote a script to go through all of our pull requests to quantify how many bors commands appear in review comments. Here are the results:

Repo	Review comment commands
rust-lang/cargo	0
rust-lang/rust-clippy	0
rust-lang-nursery/compiler-builtins	0
rust-lang-nursery/crater	0
rust-lang/hashbrown	0
rust-lang/libc	0
rust-lang/regex	0
rust-lang/rls	0
rust-lang/rust	1
rust-lang/rustlings	0
rust-lang/rustup.rs	0

So, the only time this feature has been used (that I can find; I won't rule out that my code was not 100% perfect) was this 5 Dec 2018 comment.

In the spirit that the easiest code to maintain is no code, I propose that we remove support for this rarely used feature. The total cost of supporting it is pretty low, but the utility of this function appears even lower (one use in 5 years).

I believe it would mostly come down to removing the following two parts of the code:

homu/homu/main.py

Lines 1507 to 1522 in df43bc0

	for comment in pull.iter_comments():
	if comment.original_commit_id == pull.head.sha:
	parse_commands(
	comment.body,
	comment.user.login,
	repo_label,
	repo_cfg,
	state,
	my_username,
	db,
	states,
	sha=comment.original_commit_id,
	command_src=comment.to_json()['html_url'],
	# FIXME switch to `comment.html_url`
	# after updating github3 to 1.3.0+
	)

homu/homu/server.py

Lines 372 to 402 in df43bc0

	if event_type == 'pull_request_review_comment':
	action = info['action']
	original_commit_id = info['comment']['original_commit_id']
	head_sha = info['pull_request']['head']['sha']
	if action == 'created' and original_commit_id == head_sha:
	pull_num = info['pull_request']['number']
	body = info['comment']['body']
	username = info['sender']['login']
	state = g.states[repo_label].get(pull_num)
	if state:
	state.title = info['pull_request']['title']
	state.body = info['pull_request']['body']
	if parse_commands(
	body,
	username,
	repo_label,
	repo_cfg,
	state,
	g.my_username,
	g.db,
	g.states,
	realtime=True,
	sha=original_commit_id,
	command_src=info['comment']['html_url'],
	):
	state.save()
	g.queue_handler()

[Mark-Simulacrum]

I'm somewhat ambivalent on this. It's certainly a simplification, but it also seems somewhat odd to not support review comments as from a GH UI perspective they're quite similar. I honestly didn't think we supported them and seem to recall instances where I didn't use them as a result, so I think the statistics may be flawed in that regard.

[bryanburgers]

As I've thought about this more, it seems over-aggressive to prune a feature of pretty minimal cost. Closing.