Set target=1.9 and specify "-XDignore.symbol.file" in debian/pom.xml

Also remove 1.5 source/target levels from debian/maven.properties.

This addresses FTBFS with openjdk-10 "package sun.misc does not exist"
(Debian: #902317)

Note that the approach of specifying the javac option (as per [1])

  --add-exports jdk.unsupported/jdk.internals=sun.misc

will result in a successful compilation on the command-line, but not
in conjuction with maven-compiler-plugin, which adds --release to the
compilation and results in the following error:

  [ERROR] error: exporting a package from system module jdk.unsupported is not allowed with --release

[1] https://docs.oracle.com/javase/9/migrate/