Remove public signal check from signal prefix

[chinandrew]

Description

Removes footgun for wip signals by having the add_prefix signal not auto-add "wip_" to non-public signals.

grep -r '"wip_signal":' * shows everything is "wip_signal": "", or "wip_signal": [""], in the repo, so it's hard to say if any signals would be affected by this change. I had to update some cdc covidnet changes, not sure if those are actually wip and we need to add it to the params json

Changelog

Itemize code/test/documentation changes and files added/removed.

• Remove public_signal() function
• Have add_prefix just return the input signals if wip_signals is false or ''
• Fix cdc covidnet tests.

Fixes

• The OKRs "Remove footgun in WIP signal rename widget"

[krivard]

lgtm

covidnet is not scheduled in production, so we won't get any accidental releases there.