Remove screenshot from .github/SECURITY.md

It turns out there are different paths to get to the security page on GitHub, and one of them ends up with a broken image. In retrospect, my attempt to be newbie-friendly went overboard with this, and it was a bad idea to try to provide a screenshot because it's too prone to breakage. Let's remove it.

Author: mhucka

.github/SECURITY.md

@@ -8,14 +8,9 @@ contributions.
 Please **do not** use GitHub issues to report security vulnerabilities; GitHub
 issues are public, and doing so could allow someone to exploit the information
 before the problem can be addressed. Instead, please use the *Report a
-vulnerability* interface from the *Security* tab at the top of this GitHub
+vulnerability* button from the *Security* tab at the top of this GitHub
 repository page.
 
-<div align="center">
-<img width="75%" alt="Location of the report button on the repository page"
-    src="/.github/report-vulnerability-button.png">
-</div>
-
 Please report security issues in third-party modules to the person or team
 maintaining the module rather than this project's stewards, unless you believe
 that some action needs to be taken specifically with this project in order to