Revert "Also trigger verifiers on pull requests"
Revert "Also trigger verifiers on pull requests"

This reverts commit 59649ff0f29bd9211926c0d2d8f6e7faa9a90d79.

This turns out to double the work on each verification!
That doesn't appear to be needed:
"because a merged pull request always results in a push, you can
use the push event to accomplish your goal."
https://github.community/t/trigger-workflow-only-on-pull-request-merge/17359

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm