Don't set the LD_LIBRARY_PATH in prefix.sh

Summary:
We already have rpaths anyway and it breaks running applications from
within the build directory:
https://community.kde.org/Guidelines_and_HOWTOs/Making_apps_run_uninstalled

Reviewers: Frameworks, dfaure, ngraham, graesslin

Reviewed By: ngraham, graesslin

Subscribers: graesslin, ngraham, Build System

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D9550