Remove HTML link from plain text mail

What does this MR do?

In !33316 (merged) a few things were cleaned up in the Merge Request mail. But a HTML link, i.e. <a href...> snug in here. This change reverts that.

Screenshots

Before

John Doe3 pushed new commits to merge request <a href="http://test.host/namespace1/project1/-/merge_requests/1">!1</a>

* 54fcc214 - Add plain text README

After

John Doe3 pushed new commits to merge request !1
Merge Request URL: http://test.host/namespace1/project1/-/merge_requests/1

* 54fcc214 - Add plain text README

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