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/ripple-effect/ripple-effect.scss

@@ -7,7 +7,6 @@
 $scale-duration: 225ms;
 $fade-in-duration: 75ms;
 $fade-out-duration: 150ms;
-$opacity-duration: $fade-in-duration + $fade-out-duration;
 
 :host {
   @include position(0, 0, 0, 0);