The enum NumConnections is double defined, once in the CConnman
class and once in the clientmodel.h file. Because clientmodel.cpp
includes clientmodel.h and net.h, there is a shadow warning.
This diff removes the definition from clientmodel.h and use the
CConnman definition instead, which simplifies the logic a bit.

ninja check

Build and run bitcoin-qt. Check the number of connections displayed in
the debug window is correct.