Remove #initialize method from model classes

... in favor of initialization via constructors. It's fine to break out
a separate private initialization method, but it appears to be
unnecessary to formally model an initialization lifecycle distinct from
construction.

src/test/java/io/bitsquare/gui/main/trade/createoffer/CreateOfferPMTest.java

@@ -51,7 +51,6 @@ public class CreateOfferPMTest {
         model = new CreateOfferModel(null, null, null, null, null, null, formatter);
 
         presenter = new CreateOfferPM(model, new FiatValidator(null), new BtcValidator(), formatter);
-        presenter.initialize();
     }
 
     @Test