Remove unused solid-shade mixin

What does this MR do?

Removes unused @mixin solid-shade

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

None

Why was this MR needed?

Reduces unnecessary css

Screenshots (if relevant)

Before:

$git grep 'solid-shade' -- '*.scss'
app/assets/stylesheets/framework/mixins.scss:45:@mixin solid-shade {

After:

$git grep 'solid-shade' -- '*.scss'
(nothing)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • All builds are passing
• 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?

#21559 (closed)