Remove any gain offset on the USRPs

It neither has any noticeable benefits nor direct relation with the intended values

radio/USRP/usrp_lib.cpp

@@ -1657,6 +1657,9 @@ extern "C" {
                                       openair0_cfg[0].tune_offset);
       s->usrp->set_rx_freq(rx_tune_req, i+choffset);
       set_rx_gain_offset(&openair0_cfg[0],i,bw_gain_adjust);
+      for (int chain_index = 0; chain_index < 4; chain_index++) {
+        openair0_cfg[0].rx_gain_offset[chain_index] = 0.0;
+      }
       ::uhd::gain_range_t gain_range = s->usrp->get_rx_gain_range(i+choffset);
       // limit to maximum gain
       //double gain=openair0_cfg[0].rx_gain[i]-openair0_cfg[0].rx_gain_offset[i];