Disable initdb cancellation (#6451)

## Problem

The initdb cancellation added in #5921 is not sufficient to reliably
abort the entire initdb process. Initdb also spawns children. The tests
added by #6310 (#6385) and #6436 now do initdb cancellations on a more
regular basis.

In #6385, I attempted to issue `killpg` (after giving it a new process
group ID) to kill not just the initdb but all its spawned subprocesses,
but this didn't work. Initdb doesn't take *that* long in the end either,
so we just wait until it concludes.

## Summary of changes

* revert initdb cancellation support added in #5921
* still return `Err(Cancelled)` upon cancellation, but this is just to
not have to remove the cancellation infrastructure
* fixes to the `test_tenant_delete_races_timeline_creation` test to make
it reliably pass

Fixes #6385

test_runner/fixtures/pageserver/utils.py

@@ -371,8 +371,24 @@ def tenant_delete_wait_completed(
     pageserver_http: PageserverHttpClient,
     tenant_id: TenantId,
     iterations: int,
+    ignore_errors: bool = False,
 ):
-    pageserver_http.tenant_delete(tenant_id=tenant_id)
+    if not ignore_errors:
+        pageserver_http.tenant_delete(tenant_id=tenant_id)
+    else:
+        interval = 0.5
+
+        def delete_request_sent():
+            try:
+                pageserver_http.tenant_delete(tenant_id=tenant_id)
+            except PageserverApiException as e:
+                log.debug(e)
+                if e.status_code == 404:
+                    return
+            except Exception as e:
+                log.debug(e)
+
+        wait_until(iterations, interval=interval, func=delete_request_sent)
     wait_tenant_status_404(pageserver_http, tenant_id=tenant_id, iterations=iterations)