Revert "Fixes #3929 - Set /etc/default/jetty as a config file (will not erase it silently durign upgrade)"

This reverts commit 35ec9668a77de4bf49c4c10e68679e243a6e8710.
Refs #3929 - Use another /etc/default file for rudder-server-root and source it into /etc/default/jetty in order to configure allocation of RAM to JVM from /etc/default/rudder-server-root