Remove redundant mixins

What does this MR do?

Removes the following redundant mixins

@mixin box-shadow($shadow) {
  box-shadow: $shadow;
}

@mixin border-radius($radius) {
  border-radius: $radius;
}

Are there points in the code the reviewer needs to double check?

Just need to make sure the refactor didn't break anything. Last time I did something like this, we faced an edge case where values .5 would pass correctly in mixins but not when passed directly to the property.

Why was this MR needed?

• Reduces redundant code which leads to happier developers 😄

Screenshots (if relevant)

None

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #22012 (closed)