Remove reference to LXDE.

Summary:
Introduction to Lubuntu mentions that Lubuntu team take care of LXDE packages in
LTS releases of Lubuntu. With 18.04 EOL in April 2021, this is no longer the case.

Test Plan: Manual compiles

Reviewers: lynorian

Reviewed By: lynorian

Differential Revision: https://phab.lubuntu.me/D112