Delete redundant docs link from "Linked items"

What does this MR do and why?

Delete redundant docs link from "Linked items".

More in #510357 (closed) and &16344 (comment 2269417958).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Make sure the work_items feature flag is enabled.
2. Create a task: https://docs.gitlab.com/ee/user/tasks.html#create-a-task
3. Observe the "Linked items" widget.

Related to #510357 (closed)