fix: Do not setup SimpleXmppStringprep twice.

[bgrozev]

JxmppContext.java already does it. Calling it from XmppStringPrepUtil can result in replacing the desired instance if it's done prior to XmppStringPrepUtil being loaded, e.g.:

System.err.println("Initially: " + JxmppContext.getDefaultContext().xmppStringprep.getClass().getName());
JxmppContext.setDefaultXmppStringprep(RocksXmppPrecisStringprep.INSTANCE);
System.err.println("Changed to Rocks: " + JxmppContext.getDefaultContext().xmppStringprep.getClass().getName());
try {
    JidCreate.from("example.com");
} catch(Exception e) {}
System.err.println("After parsing: " + JxmppContext.getDefaultContext().xmppStringprep.getClass().getName());

Results in:

Initially: org.jxmpp.stringprep.simple.SimpleXmppStringprep
Changed to Rocks: org.jxmpp.stringprep.rocksxmppprecis.RocksXmppPrecisStringprep
After parsing: org.jxmpp.stringprep.simple.SimpleXmppStringprep

[Flowdalic]

Thanks, please squash