Remove tooltips in diff expansion in favor of title attribute

What does this MR do and why?

• Removes tooltips in diff expansion buttons (the title attribute displays the title of the button already)
• Why? These tooltips are distracting and sometimes broken (they don't always disappear and sometimes scroll away with the page-- a problem with all tooltips)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Hover over an expansion button in an MR diff

MR acceptance checklist

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