jlink fails on alpine-slim with java.desktop

jlink fails in the alpine-slim image.

```
FROM adoptopenjdk/openjdk11:alpine-slim

RUN ["jlink", "--compress=2", \
"--add-modules", "java.base,java.desktop", \
"--output", "/linked"]
```

```
$ docker build .
Sending build context to Docker daemon 6.144kB
Step 1/2 : FROM adoptopenjdk/openjdk11:alpine-slim
---> 11ec9a62b568
Step 2/2 : RUN ["jlink", "--compress=2", "--add-modules", "java.base,java.desktop", "--output", "/linked"]
---> Running in 23566d540cb3
Error: Module java.desktop not found
The command 'jlink --compress=2 --add-modules java.base,java.desktop --output /linked' returned a non-zero code: 1
```

The desktop module is reported as being present.

```
# java --list-modules |grep desk
java.desktop@11.0.4
jdk.unsupported.desktop@11.0.4
```

It might be [removing jmod files](https://github.com/AdoptOpenJDK/openjdk-docker/blob/master/11/jdk/alpine/slim-java_jmod_del.list#L19) that is breaking jlink in the slim builds.

Consider fixing `jlink` to be supported in slim images, or remove jlink and associated tools from the slim build and point people towards running jlink against the non-slim build.

[this problem might be hit if [beans](http://mail.openjdk.java.net/pipermail/jigsaw-dev/2018-March/013654.html) are used.]