java_gen: fix java_gen on NFS

java_gen was removing the old working directory, with the loxigen
infrastructure still having a file handle open. This doesn't work
on NFS.
Merge branch 'master' of github.com:floodlight/loxigen
Merge into master from pull request #93:
java_gen: fix java_gen on NFS (https://github.com/floodlight/loxigen/pull/93)