[DEPR]: Remove legacy File support from HTML XBlock

[feanil]

See this PR for details: openedx/openedx-platform#14163

The HTMLModule had some compatibility code meant for XML authored courses from the Fall of 2012, before we changed the file naming conventions. All courses created from 2013 onwards have used the "new" file naming scheme, so this code should no longer be necessary.

There needs to be some follow up to make sure it won't break some old MIT courses.

Additional Info

Original Jira Issue: https://openedx.atlassian.net/browse/DEPR-45

[dianakhuang]

We're going to try to run a search on MIT courses to see if any of them are using these file extensions: .html.html or .html.xml

[ormsbee]

FWIW, Old Mongo courses are now dead, so if this shows up in one of those courses, it's fine.

[robrap]

[inform] This DEPR is in the Draft state. It can't move forward without a DEPR coordinator to help update it to match the updated DEPR template. No action is required, unless anyone wants to move this out of Draft.

[dianakhuang]

@pdpinch to rerun the search for possible issues with this.

[dianakhuang]

There aren't any usages of this in MITx as of right now, so we believe this can move forward.