Bug report template: remove empty 'title' field.

Mysteriously, GitHub can't parse it if it's an empty string rather than
just missing.

Signed-off-by: Avery Pennarun <apenwarr@tailscale.com>

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -1,6 +1,5 @@
 name: Bug report
 description: File a bug report
-title: ""
 labels: [needs-triage, bug]
 body:
   - type: markdown