forward_update_8201() leaves stale forward_filter_format setting in active config

Problem/Motivation

By request, I'm testing 8.x-2.3-rc1.

In #2969418: Allow a separate filter format for plain text vs. HTML emails we introduced forward_update_8201() to split the forward filter setting into an HTML vs. plain text setting.
Unfortunately, that update leaves the original setting still in config:

forward_filter_format: email_html

Since that's not defined in the schema, we get schema validation errors when you run tests after upgrading.
It's also leaking stuff into config we're no longer using, leading to possible confusion and human error.

Proposed resolution

Add a new update to purge the dead setting from config and re-save.

Remaining tasks

1. Do it.
2. Test.
3. Commit.

User interface changes

None.

API changes

None.

Data model changes

None, really. Fixes a mismatch between actual config and expected config schema.

Release notes snippet

Probably none. TBD.