[Feature flag] Remove frontend for Holt Winters data forecast

Problem

We experimented with Holt Winters for better forecasting on the Deployment Frequency chart but we are no longer investing in that direction and the code needs to be removed.

Proposal

Remove the frontend code related to the Holt Winters forecasting feature flag [Feature flag] Holt Winters data forecast for d... (#419209)

Be sure to leave the linear regression forecasting in place.