Remove vulnerability counter from security tab

What does this MR do?

This MR removes the vulnerability count from the Security tab in the pipeline view. The counter has been deemed superfluous because:

• It does not reflect the severity of the vulnerabilities (i.e. 1 critical + 9 low = 10 and 9 critical + 1 low = 10)
• When filtering vulnerabilities in the security dashboard, the counter would be updated with the filtered item count, instead of statically displaying the overall total

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Closes #13978 (closed)