Remove broken Security Vulnerability Reporting link (#14)

We use the org-wide security reporting document instead, which is visible on Github under the
'Security' tab on the README. This seems sufficiently discoverable and will be updated automatically
for all repos in the org, so we don't have to manage it separately here.

Author: jacquesh

README.md

@@ -7,7 +7,6 @@
 - [Contributions](#contributions)
 - [License](#license)
 - [Code of Conduct](#code-of-conduct)
-- [Security Vulnerability Reporting](#security-vulnerability-reporting)
 
 ## Rationale