ci(check-whitespace): stop requiring a read/write token

ci(check-whitespace): stop requiring a read/write token

As part of some recent security tightening, GitHub introduced the
ability to configure GitHub workflows to be run with a read-only token.
This is much more secure, in particular when working in a public
repository: While the regular read/write token might be restricted to
writing to the current branch, it is not necessarily restricted to
access only the current Pull Request.

However, the `check-whitespace` workflow threw a wrench into this plan:
it _requires_ write access (because it wants to add a PR comment in case
of a whitespace issue).

Let's just skip that PR comment. The user can always click through to
the actual error, even if it is slightly less convenient.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>