Remove build-time dependency on plugin.jar from FX jake build

Once JDK-8132744 is fixed we will no longer have a runtime dependency on the jdk.plugin module. This JIRA is a follow-up to remove the compile-time dependency on plugin.jar. In order to do this, we will either need to switch to JDK 9 build 109 (or 110), which includes the new netscape.javascript package in the runtime image, or we will need to make the netscape.javascript class files available at build time and copy them into jfxrt.jar (but not the modular_sdk, since we must not deliver these into the JDK jigsaw build (they were removed by JDK-8090136).

This must be done in the FX jake sandbox prior to m3 integration.

Given the tight timeline I might implement the second solution this week, and then switch to the first solution next week if we are able to switch our mainline 9 builds to using JDK 9+109.