[feature] drop JNLP connection stuff, and switch to pure xmlrpc solution

[duncdrum]

Is your feature request related to a problem? Please describe.
Getting rid of the jar download requirement where possible, should be less error prone, and ultimately beneficial to the user.

Describe the solution you'd like
Managing collection of jars locally, should not be necessary.

Describe alternatives you've considered
Do nothing, if it works JNLP still works. What takes over with the current responsibility is secondary, xmlrpc, xmldb, rest, …

Additional context
see #5545