util: change GetWarnings parameter to bool

GetWarnings() changes the format of the output warning string based on a
passed-in string argument that can be set to "gui" or "statusbar".
Change the argument to a bool:

- there are only two types of behaviour, so a bool is a more natural
argument type
- changing the name to 'verbose' does not set any expectations for the
how the calling code will use the returned string (currently,
'statusbar' is used for RPC warnings, not a status bar)
- removes some error-handling code for when the passed-in string is not
one of the two strings expected.

src/rpc/net.cpp

@@ -522,7 +522,7 @@ static UniValue getnetworkinfo(const JSONRPCRequest& request)
         }
     }
     obj.pushKV("localaddresses", localAddresses);
-    obj.pushKV("warnings",       GetWarnings("statusbar"));
+    obj.pushKV("warnings",       GetWarnings(false));
     return obj;
 }