Drop unused `.bg-body-emphasis`

Description

This PR suggests to drop .bg-body-emphasis generation since it is not used anywhere and is not even displayed in our Utilities > Background > Background color documentation.

If we need to still create it for consistency:

• It must be added to Utilities > Background > Background color documentation
• --bs-emphasis-bg and --bs-emphasis-bg-rgb must be defined which is not the case right now

Type of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• All new and existing tests passed