Remove unused and problematic "dropdown-menu" ident from css

What does this MR do and why?

Remove dropdown-menu css width property that was causing the notification dropdown to be illegible.

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MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Have you checked this doesn't interfere/conflict/duplicate someone elses work?
• Have you fully tested your changes?
• Have you resolved any lint issues?
• Have you assigned a reviewer?
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