Fix(Dropdown): Made the titleText optional in propTypes

[Thamjith]

Closes #17843

This PR makes the titleText an optional property in the propTypes, since we are only showing titleText conditionally.

Changelog

Changed

• Made the titleText optional in propTypes.

Testing / Reviewing

• Remove the titleText property from in "packages/react/src/components/Dropdown/Dropdown.stories.js"
• Navigate to the packages/react directory using cd packages/react and run yarn storybook
• Check the console in browsers's developer tools

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[Thamjith]

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[Thamjith]

recheck