Composer: don't run php artisan optimize before a build

Summary:
Cases where php artisan optimize is needed, especially when opcache
is enabled on production servers are limited enough to be a choice to
made by each deployment, not automatically done.

We so remove these step from composer install/update.

For CI purpose, that will speed up builds, ref T1003.

Test Plan: Run a build on CI infrastructure.

Reviewers: dereckson

Tags: Continous integration and delivery

Maniphest Tasks: T1003

Differential Revision: https://devcentral.nasqueron.org/D645