chore(zizmor): drop the local policy copy, the reusable supplies it (#261)

Removes `.github/zizmor.yml`.
[netresearch/.github#339](netresearch/.github#339)
makes the zizmor reusable fetch the organisation policy at run time when
a repo has no file of its own, and drops it from all five templates, so
this copy no longer has a job.

It is not harmless to leave behind. A local file takes precedence over
the fetched one, so this repository would keep running an old policy the
next time the shared one actually changes — which is exactly how a
two-line policy ended up needing a pull request in 54 repositories to
reword a comment.

The scan result does not change: the same policy is applied, from one
place instead of 54. A repository that genuinely needs a different
policy opts out by keeping its own `.github/zizmor.yml`; this one was
byte-identical to the template, which was verified before deleting.

This pull request previously carried the comment rewording from
[#336](netresearch/.github#336). That became
pointless once the file itself moved, so the branch was rewritten to
delete it instead.

Author: CybotTM