revert: revert to upstream approach to use logo url

[mudassir-hafeez]

What are the relevant tickets?

Following up https://github.com/mitodl/hq/issues/3337#issuecomment-2357798696.

Description (What does it do?)

This PR reverts to the upstream approach for using the logo URL, as the upstream PR and its backport have been merged.

Screenshots (if appropriate):

How can this be tested?

• Ensure the mitxonline-theme is configured.
• Set LOGO_URL_PNG_FOR_EMAIL in the settings/private.py file.
• Verify that emails sent via batch enrollment use the value set in LOGO_URL_PNG_FOR_EMAIL

Additional Context

• We might not need to manually set LOGO_URL_PNG_FOR_EMAIL, as it appears to be already set here for all instances.

[asadali145]

LGTM!