Update Gradle to 4.9
Summary: Remove desktop file, as it's unnecessary for a primarily terminal-based program. Upgrade to the latest version that fully supports being built with JDK 8.
Test Plan: Build openjfx-8 and a moderately sized personal Gradle project
Reviewers: Triage Team, DataDrake
Reviewed By: Triage Team, DataDrake
Differential Revision: https://dev.getsol.us/D7512