Remove unnecessary background styles from language picker

[aduth]

🛠 Summary of changes

Removes a few language picker styles leftover after #10098, where prior to that pull request the expander icon was implemented as a background image.

These styles do nothing because the background image is always removed:

identity-idp/app/assets/stylesheets/components/_language-picker.scss

Lines 46 to 49 in ddf78f0

	&.usa-accordion__button[aria-expanded='false'],
	&.usa-accordion__button[aria-expanded='true'] {
	background-image: none;
	}

This was noticed as part of adapting the styles to the brochure site in GSA-TTS/identity-site#1219 .

I also simplified multiple padding styles to a single shorthand Y X format (reference).

📜 Testing Plan

Verify no regression in the appearance of the language picker, notably the expander icon, on both desktop and mobile.