Fix understrap_adjust_body_class

[IanDelMar]

Description

This PR lets understrap_adjust_body_class() return the unfiltered parameter $classes.

Motivation and Context

Although this function has been deprecated in v0.9.4 it might break the body classes if a user with an older version using of Understrap understrap_adjust_body_class() upgrades to a newer version.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• composer cs:check has passed locally.
• composer lint:php has passed locally.
• I have read the CONTRIBUTING document.