[LOOM-1280][BpkDatePicker] Remove useless styling

[metalix2]

Reviewed the Styling applied to the BpkDatePicker and found it made no effect to the Calendar component. Tested across chrome, firefox and safari on varying mobile widths. Looked up the history of the changes and couldn't find much to indicate the reason it was added.

Remember to include the following changes:

• Ensure the PR title includes the name of the component you are changing so it's clear in the release notes for consumers of the changes in the version e.g [KOA-123][BpkButton] Updating the colour
• README.md (If you have created a new component)
• Component README.md
• Tests
• Storybook examples created/updated
• Type declaration (.d.ts) files updated
• For breaking changes or deprecating components/properties, migration guides added to the description of the PR. If the guide has large changes, consider creating a new Markdown page inside the component's docs folder and link it here

[github-actions]

	Warnings
⚠️	Package source files (e.g. packages/package-name/src/Component.tsx) were updated, but type files weren't. Have you checked that no types have changed?

Browser support

If this is a visual change, make sure you've tested it in multiple browsers.

Generated by 🚫 dangerJS against a8895d5

[github-actions]

Visit https://backpack.github.io/storybook-prs/3388 to see this build running in a browser.

[github-actions]

Visit https://backpack.github.io/storybook-prs/3388 to see this build running in a browser.

[mungodewar]

LGTM