fix: remove gradient on legend slider

[severinlandolt]

Description

This PR removes the gradients on when legend slider is enabled.

Related issue(s)

What kind of change does this PR introduce? (check at least one)

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New Feature (non-breaking change which adds functionality)
• [ ] New Feature (BREAKING CHANGE which adds functionality)
• [x] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and migration path for existing applications:

How has this been tested?

Storybook

Screenshots (if appropriate):

The PR fulfils these requirements:

• [x] It's submitted to the main branch
• [ ] When resolving a specific issue, it's referenced in the related issue section above
• [ ] My change requires a change to the documentation. (Managed by Tremor Team)
• [ ] I have added tests to cover my changes
• [ ] Check the "Allow edits from maintainers" option while creating your PR.
• [ ] Add refs #XXX or fixes #XXX to the related issue section if your PR refers to or fixes an issue.
• [x] By contributing to Tremor, you confirm that you have read and agreed to Tremor's CONTRIBUTING.md guideline. You also agree that your contributions will be licensed under the Apache License 2.0 license.

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Name	Status	Preview	Updated (UTC)
tremor-storybook	✅ Ready (Inspect)	Visit Preview	Feb 20, 2024 11:02pm

[github-actions[bot]]

:tada: This PR is included in version 3.14.1-beta.1 :tada:

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: