Prune /var/www/html on shellserver role

Summary:
Ref T1325.

Clean-up for 51f54c48698f.

Test Plan: Check it's well no-op on Eglide, browsing https://www.eglide.org.

Reviewers: dereckson

Maniphest Tasks: T1325

Differential Revision: https://devcentral.nasqueron.org/D1256