Prune unused webserver-content index generator

Summary:
Previously, roles/webserver-content/init.sls was
generated by an utility script, still in repository.

This script isn't used anymore, as pillar defines
directly the list of .sls to include.

This is a follow-up for 2f08293fec4b.

Ref T2123

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2123

Differential Revision: https://devcentral.nasqueron.org/D3665