[Qt] minor optimisations in peertablemodel

- remove an unneeded include of net.h in peertablemodel.cpp
- add const after size() in PeerTablePriv
- remove 2x unneeded else in functions
- replace a (int) typecast by (QVariant) to use Qt style
- remove unneeded include of peertablemodel.h from rpcconsole.cpp

src/qt/rpcconsole.cpp

@@ -7,7 +7,6 @@
 
 #include "clientmodel.h"
 #include "guiutil.h"
-#include "peertablemodel.h"
 #include "platformstyle.h"
 
 #include "chainparams.h"