Remove Oracle JDK 8 build.

[comcast-jonm]

This build was working when the default Ubuntu distribution was Trusty (14.04), but Trusty was EOL'ed as of April 2019. Given that Oracle changed its licensing schemes for its JDK, it seems simplest to remove the Oracle JDK build, rather than retrofitting the build and pinning it with dist: trusty to an EOL distro.

For further context, see:

• https://travis-ci.community/t/install-of-oracle-jdk-8-failing/3038/8
• https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment