infra: remove provision-gitea-runner.sh (now lives with the infra team)

The runner-provisioning script has been handed off to the infra
agent, who will own it going forward. ci.yml's header comment is
updated to point at "the infra team owns the script" rather than
the in-repo path, but the runner expectations themselves stay the
same — workflows still rely on the persistent volumes, pre-cloned
actions, and host-installed golangci-lint that any compliant
provisioning produces.

.gitea/workflows/ci.yml

@@ -3,10 +3,11 @@
 # Notes for anyone editing this file:
 #
 # Self-hosted runner expectations
-#   The Gitea runners are provisioned via scripts/provision-gitea-runner.sh.
-#   That script bind-mounts persistent host volumes for /root/go/pkg/mod
-#   (GOMODCACHE), /root/.cache/go-build (GOCACHE), and /root/.cache/act
-#   (action clones) into every job container. As a result:
+#   The Gitea runners are provisioned out-of-band (the infra team owns
+#   the script). Each runner host bind-mounts persistent volumes for
+#   /root/go/pkg/mod (GOMODCACHE), /root/.cache/go-build (GOCACHE), and
+#   /root/.cache/act (action clones) into every job container. As a
+#   result:
 #     * `cache: true` on actions/setup-go is intentionally OMITTED — the
 #       action would otherwise tar/untar GOMODCACHE+GOCACHE through the
 #       Gitea cache backend on every job, undoing the host-volume cache