Remove an obsolete comment about Mercurial SSH error behavior

Summary:
Depends on D18855. Ref T13036. This comment no longer seems to be accurate: anything we send over stderr is faithfully shown to the user with recent clients.

From this document, the missing sauce may have been:

A generic error response type is also supported. It consists of a an error
message written to ``stderr`` followed by ``\n-\n``. In addition, ``\n`` is
written to ``stdout``.

That is, writing "\n" to stdout in addition to writing the error to stderr. However, this no longer appears to be necessary.

I think the modern client behavior is generally sensible (and consistent with the behavior of Git and Subversion) so this probably isn't a bug or me making a mistake.

Test Plan: With a modern client, threw some arbitrary exception during execution. Observed a helpful message on the client with no additional steps.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13036

Differential Revision: https://secure.phabricator.com/D18856