Refactor Pipeline Show page object

What does this MR do and why?

Continue work on phasing out data-qa-selector and replace with data-testid. This MR works on Page::Project::Pipeline::Show.

Also removed unused selectors and their corresponding methods.

Team task gitlab-org/quality/quality-engineering/team-tasks#1826 (closed)

OKR https://gitlab.com/gitlab-com/gitlab-OKRs/-/work_items/4103

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