Alert: Remove invalid and redundant alert height style

[aduth]

Summary

Removes an unnecessary assignment of height styling in the add-alert-icon mixin.

• This produces invalid styles because $theme-alert-icon-size is a unitless number (e.g. 2) and should be wrapped with the unit function
• This is redundant anyways because the height is already assigned by the inclusion of add-color-icon mixin above using the height value from $this-icon-object

Testing and review

Observe the issue by inspecting the ::before pseudo-element of .usa-alert__body in the Alert component preview page.

The changes proposed in this pull request will cause the second (invalid) highlighted height style to be removed.

[mahoneycm]

Confirming this effectively removes the redundant height definition from the alert icon!

The icon height is still defined through $theme-alert-icon-size via $this-icon-object directly above in the CSS. Removing the height definition from $theme-icon-object also does not use the height defined within the &::before selector so we can safely remove.

[aduth]

This issue might not actually be so harmless in all cases, since some CSS post-processors may try to correct the style by adding a missing unit.

For example, Lightning CSS will change the 4 to 4px, which will have an impact on the rendered style of the alert.

$ node -p "require('lightningcss').transform({ code: Buffer.from('body { height: 4; }') }).code.toString()"
body {
  height: 4px;
}

Edit: See example impact at 18F/identity-idp#8093 (comment)

[mahoneycm]

Created change log PR for site: uswds/uswds-site#2094