Remove ENV vars for ZAP config file from DAST docs

What does this MR do?

This MR removes the ENV variables for DAST that allow users to use a ZAP config file. The config file is buggy and we'll replace anything from it that is valuable with our own feature work in the future.

MR that removes the variables from the code: gitlab-org/security-products/dast!185 (merged) Issue: #218808 (closed) Issue: #221138 (closed)

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