Components: Simplify CustomGradientPicker's utility props

[chad1008]

What?

When recently migrating CustomGradientPicker to TypeScript, an abstraction was used that is more complicated than it needed to be. This PR simplifies the affected types.

Why?

Simplifying these types will make them easier to parse and work with by other contributors in the future.

How?

The utility function impacted by this change uses only certain props from an existing type. The abstraction that is currently in place narrows the existing type to more closely match the expected inputs of the function, but this narrowing isn't technically necessary. To alleviate the added complexity, the types are being updated to just accept the existing type, and the extra prop(s) can be seamlessly ignored without causing any errors.

Testing Instructions

• Confirm that there are new new TS errors introduced by this change
• Confirm unit tests pass

[github-actions]

Flaky tests detected in 948e5b5.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4611585556
📝 Reported issues:

• [Flaky Test] prevent the removal of locked column block from the column count change UI #49534 in /test/e2e/specs/editor/blocks/columns.spec.js

[ciampo]

🚀

Thank you and @mcsf for always looking for ways to make our code better ❤️