Update analytics dashboard skeleton loader

What does this MR do and why?

Remove the lines=3 prop from GlSkeletonLoader because the number of lines is 3 by default.

Note that product analytics is an experimental feature behind a default off feature flag.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Verify the onboarding list item shows correctly.
4. Verify the single dashboard view by opening the Audience dashboard.

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.