Copter: clarify EKF check msg to GCS

remove "compass" from the error message because the failure can also be
caused by GPS glitches

ArduCopter/ekf_check.pde

@@ -79,7 +79,7 @@ void ekf_check()
                 Log_Write_Error(ERROR_SUBSYSTEM_EKF_CHECK, ERROR_CODE_EKF_CHECK_BAD_COMPASS);
                 // send message to gcs
                 if ((hal.scheduler->millis() - ekf_check_state.last_warn_time) > EKF_CHECK_WARNING_TIME) {
-                    gcs_send_text_P(SEVERITY_HIGH,PSTR("EKF: compass variance"));
+                    gcs_send_text_P(SEVERITY_HIGH,PSTR("EKF variance"));
                     ekf_check_state.last_warn_time = hal.scheduler->millis();
                 }
                 failsafe_ekf_event();