Remove "Customize Lesson Template" task

[donnapep]

Is your feature request related to a problem? Please describe

The Customize Lesson Template task gets few clicks (7% on self-hosted and 8% on Atomic) and doesn't seem important enough to be part of the onboarding checklist. The checklist for when Sensei Pro + Woo are activated will also have a new Create your first product task, making the checklist that much longer.

Describe the solution you'd like

Remove the Customize Lesson Template task from the onboarding checklist, and any associated usage tracking code.