Remove non-existing gitlab pages params

What does this MR do?

The doc section Global settings contains the reference to the gitlab.rb options gitlab_pages['ssl_certificate'] and gitlab_pages['ssl_certificate_key'] but seems that they do not exist:

• when wildcard DNS with TLS is used, one should set pages_nginx['ssl_certificate'] & pages_nginx['ssl_certificate_key']
• when custom domains with TLS are used, one should set gitlab_pages['cert'] & gitlab_pages['cert_key']

This MR removes gitlab_pages['ssl_certificate'] and gitlab_pages['ssl_certificate_key'] from the docs.

Screenshots (strongly suggested)

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team