Discontinue building the image based on OpenJDK JRE 7

[vutny]

Although OpenJDK still maintains some JRE/JDK 7 code updates, the latest and greatest ready-to-consume builds are exclusively provided to RedHat subscribers.

Last time the binary package of 7u231 version was available in Debian "jessie" (oldoldstable) at 2019-08-22 (7u231). But there are still 14 pending security vulnerabilities.
The official Docker images of openjdk:7-jre have not been updated since the merge of docker-library/openjdk#322 almost 6 months ago.

I think it is the time to stop supplying Jetty 9.2 images based on outdated OpenJDK and base operating system. The latest release was 9.2.29.v20191105, which works on JRE 8, but the EOL was declared last year.

Considering all of that I think we could safely remove 9.2-jre7 directory from this repo. The images are still on Docker Hub for whoever needs them.

[gregw]

Fixed with #118