Revert "[libc++] Try handling spurious cancellation in the mainline CI restarter"

This reverts commit ecbf64d, since that seems to cause the CI to
restart-loop instead of doing what it should.

Author: ldionne

.github/workflows/libcxx-restart-preempted-jobs.yaml

@@ -33,7 +33,7 @@ jobs:
         with:
           script: |
             const failure_regex = /Process completed with exit code 1./
-            const preemption_regex = /(The runner has received a shutdown signal)|(The operation was canceled)/
+            const preemption_regex = /The runner has received a shutdown signal/
 
             const wf_run = context.payload.workflow_run
             core.notice(`Running on "${wf_run.display_title}" by @${wf_run.actor.login} (event: ${wf_run.event})\nWorkflow run URL: ${wf_run.html_url}`)