Remove PcapTxTtagStats::handle_ member

PcaptxTtagStats inherits from PcapSession which already includes a protected
handle_ member.

This removal was likely left off when PcapTxTtagStats started inheriting from
PcapSession.

server/pcaptxttagstats.h

@@ -44,7 +44,6 @@ private:
 
     QString device_;
     bool isDirectional_{true};
-    pcap_t *handle_{nullptr};
     volatile State state_{kNotStarted};
     volatile bool stop_{false};