chore: remove unused sass variables (#28363)

Issue number: N/A

---------

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## What is the current behavior?
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This project has several unused Sass variables still in the code base.
The team would like to remove these.

## What is the new behavior?
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this PR. -->

- Removed unused Sass variables


## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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and migration path for existing applications below. -->


## Other information

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screenshots of how the component looks before and after the change. -->

The original scope of this ticket was for checkbox only, but many other
components had unused sass variables, so I decided to tackle everything
all at once.

Since these variables are not used anywhere:

1. The build should pass
2. There should be no screenshot diffs

core/src/components/modal/modal.ios.vars.scss

@@ -3,8 +3,5 @@
 // iOS Modals
 // --------------------------------------------------
 
-/// @prop - Background color for the modal
-$modal-ios-background-color:          $background-color !default;
-
 /// @prop - Border radius for the modal
 $modal-ios-border-radius:             10px !default;

core/src/components/modal/modal.md.vars.scss

@@ -3,9 +3,6 @@
 // Material Design Modals
 // --------------------------------------------------
 
-/// @prop - Background color for the modal
-$modal-md-background-color:       $background-color !default;
-
 /// @prop - Box shadow color of the alert
 $modal-inset-box-shadow-color:    rgba(0, 0, 0, .4) !default;