Prune newt and ncurses extensions
Summary:
The migration commit to PHP 7.1.0 claims:
"Some unmaintained PECL extensions, newt and ncurses are removed."
Yet, these two extensions was still in Dockerfile.
This change takes care to the actual pruning.
This is a follow-up for b880b97b230f
Test Plan: Build image
Reviewers: Sandlayth, dereckson
Reviewed By: dereckson
Subscribers: Sandlayth
Differential Revision: https://devcentral.nasqueron.org/D870