Remove invitation to concluded pilot test

[jugglinmike]

No description provided.

[zcorpan]

This PR fails both CI checks. Unclear to me why.

[jugglinmike]

More information about the failure:

/usr/bin/git -c protocol.version=2 fetch --no-tags --prune --progress --no-recurse-submodules --depth=1 origin +refs/heads/docs-remove-outdated-invitation*:refs/remotes/origin/docs-remove-outdated-invitation* +refs/tags/docs-remove-outdated-invitation*:refs/tags/docs-remove-outdated-invitation*
The process '/usr/bin/git' failed with exit code 1

I submitted this pull request from a fork of the W3C-owned repository. It's tough to say from the output alone (which, for those unfamiliar, is available by clicking the button labeled "0 / 2 checks OK"), but the continuous integration process may be incorrectly attempting to fetch the fork's branch as if it were present in the W3C-owned repository. The CI process may not be configured to support contributions issued from forks. In the past month, just two other pull requests have been submitted like that (gh-434 and gh-441), and they both exhibit the same error.