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