Remove requirement to create PRs from calendar repo

The calendar repository is used to maintain the calendar events for
various teams.

For some teams, it's important that the friction for adding or
updating calendar events be low, as otherwise team members start
looking for or using alternate calendaring solutions.  Keeping this
friction as low as possible is probably more important, in this case,
than the benefits that we're getting from requiring PRs as part of the
branch protection.  So let's remove that requirement.

The other option here would be to instead add `required-approvals = 0`
to the branch protection.  This would require creating PRs but would
avoid the need for a separate reviewer.

This is an option, but it would still increase the friction a bit, and
it therefore seems net-better to just remove the need to create PRs.
Many other repositories here operate without branch protection at all
without that creating any substantial problems.  In cases where
someone thinks that something deserves a PR, e.g. so it can be
discussed, that person can still of course create one.  But we don't
need to require it.

For a calendar, the speed at which it can be updated is often the most
important property.

repos/rust-lang/calendar.toml

@@ -19,3 +19,4 @@ wg-async = "maintain"
 
 [[branch-protections]]
 pattern = "main"
+pr-required = false