Remove obsolete code for date-pickers of Productivity Analytics

The following discussion from !17933 (merged) should be addressed:

• @djensen started a discussion: (+1 comment)

  Is Time.zone.now configured to return the User's timezone? If not, I worry this could be misleading. So if there's no easy way to get the User's timezone, I think we should use Time.now.

FE for Productivity Analytics was updated to use explicit from & to datepickers, so we can remove support of special Xdays values on API side.