github: Remove vulnerability.md (#21894)

This type is automatically offered by github after changing to the new style and a security.md being present

.github/ISSUE_TEMPLATE/vulnerability.md

----
-name: Report a vulnerability
-about: There is a bug in go-ethereum that can be exploited
-title: ''
-labels: 'type:security'
-assignees: ''
----
-
-Please do not submit these in this public issue tracker!
-
-To find out how to disclose a vulnerability in Ethereum visit https://bounty.ethereum.org or email bounty@ethereum.org.
-
-Please read [Reporting a vulnerability](https://github.com/ethereum/go-ethereum/security/policy#reporting-a-vulnerability) for more information.