Remove custom camera icon implementation

[aduth]

Refactoring follow-up to #6212 (comment)

Why: As a simplification, since we can use it directly from the design system.

Testing Instructions:

1. Have both the IdP and an instance of a local sample app running
2. Start at http://localhost:9292
3. Under "Options" in the top-right, select "IAL 2 (strict)"
4. Click "Sign in"
5. Complete proofing flow up to document capture
6. Upload photos of document
7. Click "Continue"
8. Grant access to your camera
9. Observe camera icon appears same as before

[aduth]

For reference:

identity-idp/app/assets/stylesheets/components/_icon.scss

Lines 3 to 19 in 883e767

	// Upstream: https://github.com/uswds/uswds/pull/4493
	.usa-icon {
	.usa-button > &:first-child {
	// Note: This diverges from the upstream pull request in a couple ways:
	// 1. There should not be any margins offsetting to account for line height, since Login.gov
	// Design System normalizes button line height to 1.
	// 2. Float is replaced by `vertical-align`, since otherwise it will have the effect of having
	// the icon appear to the far edge of the button, rather than next to the text.
	vertical-align: bottom;
	margin-right: 0.25rem;
	}
	.usa-button:not(.usa-button--unstyled) > &:first-child {
	margin-left: -1 * $icon-min-padding;
	margin-right: #{0.5rem - px-to-rem($icon-min-padding)};
	}
	}

[zachmargolis]

LGTM