[dialer] Simplify checking for call in progress

The connection will always be existing, unless the network is down
at which point the channel is already invalid. No point for checking
for it then.

dialer/plugin/tp-caller.cpp

@@ -83,7 +83,7 @@ void TpCaller::dial(const QString &number)
 
 bool TpCaller::callInProgress()
 {
-    return m_callChannel && m_callChannel->isValid() && m_callChannel->connection();
+    return m_callChannel && m_callChannel->isValid();
 }
 
 void TpCaller::hangUp()