Remove coverage percentage regex config from coverage artifact docs

What does this MR do and why?

This MR removes some configuration from example docs because it is related to the coverage artifact feature being described, but is a separate thing. We talked about this, and it might be confusing to configure them both at the same as as an example.

Discussion of this with the grouppipeline security and Technical Writing in Adding the `coverage` regex configuration optio... (gitlab-org/ci-sample-projects/coverage-report#2 - closed)

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