chore(datetime): remove unused code (#28502)

Issue number: N/A

---------

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## What is the current behavior?
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While evaluating step colors for high contrast themes I discovered that
this code is not actually applied anywhere.

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

- Remove unused code

## 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. -->

Author: liamdebeasi

core/src/components/datetime/datetime.md.scss

@@ -136,7 +136,3 @@
 
   justify-content: flex-end;
 }
-
-:host .datetime-view-buttons ion-button {
-  color: $text-color-step-200;
-}