Remove obsolete comment in cmmOffset

Authored by nomeata.

Description

Remove obsolete comment in cmmOffset

Summary:
as obviously, the code does _not_ adhere to the comment, and yet the
compiler does _not_ go into an infinite loop.

Test Plan: Run validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D719