Remove custom LBI color scale

[huddlej]

Description of proposed changes

Closing an ancient TODO to remove the hardcoded color scale for LBI, since we can define custom scales now.

Example

See deployment of H3N2 HA tree colored by LBI.

Related issue(s)

Closes #805

Checklist

• Checks pass
• If making user-facing changes, add a message in CHANGELOG.md summarizing the changes in this PR
• (to be done by a Nextstrain team member) Create preview PRs on downstream repositories.

[jameshadfield]

👍 do you think it's worth seeing which (current core) datasets use LBI, or are you fairly confident the ones which do already define a custom scale?

Ideally we'd add a changelog entry to the PRs, although I do try to add missing stuff to the changelog immediately prior to release

[huddlej]

I can add a change log note. Just checked the LBI for H3N2 HA 2y build in the deployment and it looks good. One other change that happened since 2018 is that LBI values got normalized to a max of 1, so the current auto-scale looks reasonable and generally should unless something goes terribly wrong with the LBI calculation.