Remove --color-tan, --color-tan-light, --color-tan-dark, and --color-tan-darker variables

[sandbergja]

What maintenance needs to be done?

Remove the following variables from both CSS and SCSS:

• --color-tan
• --color-tan-light
• --color-tan-dark
• --color-tan-darker

--color-tan-lighter is actually in use, so we should keep it.

Level of urgency

• High
• Moderate
• Low

Why is this maintenance needed?

Since these variables are not used, we can simplify.

[sdellis]

I appreciate this ticket, but want to make sure that there is a process for redefining a palette that goes beyond whether or not colors (or other atomic design tokens) are being used or not.

Blocked by #362 .