Remove "--lintall" and "--only-new" flags to "arc diff"

Summary:
Ref T13544. These flags change the behavior of the "arc lint" subprocess.

I believe there is no reason to ever use "arc diff --lintall". If you want to find extra warnings to fix, you can use "arc lint --lintall" to express this intent.

Use of "arc diff --only-new" almost certainly means your linters are raising messages at "error" severity which should instead be raised at "warning" severity. If you only care about fixing a particular type of error in changed code, it should be raised as a "warning". The correct remedy is to adjust the severity, not use "--only-new", which is a very broad, slow, complicated hammer.

Test Plan: Searched for "lintall" and "only-new" in this workflow. These flags still exist in "arc lint", but may be changed in the future. Generated this change.

Maniphest Tasks: T13544

Differential Revision: https://secure.phabricator.com/D21298