Build/Test Tools: Trim performance workflow from 6.7-6.4.

[61736], [61801], [61082], and [61083] removed the `performance.yml` workflow from the 6.7-6.4 branches as part of #64083.

The workflow also needs to be removed from the workflow responsible for testing old branches to avoid failures when attempting to trigger a `workflow_dispatch` event on a non-existent workflow file.

See #64227, #64083.

git-svn-id: https://develop.svn.wordpress.org/trunk@62035 602fd350-edb4-49c9-b593-d223f7449a82