upeksonly: Remove callback support after killing transfers

This seems to have been unused even before the v2 port.

libfprint/drivers/upeksonly.c

@@ -52,9 +52,6 @@ enum sonly_kill_transfers_action {
 
   /* iterate a SSM to the next state */
   ITERATE_SSM,
-
-  /* call a callback */
-  EXEC_CALLBACK,
 };
 
 enum sonly_fs {
@@ -94,11 +91,7 @@ struct _FpiDeviceUpeksonly
 
   enum sonly_kill_transfers_action killing_transfers;
   GError                          *kill_error;
-  union
-  {
   FpiSsm                          *kill_ssm;
-    void    (*kill_cb)(FpImageDevice *dev);
-  };
 
   struct fpi_line_asmbl_ctx        assembling_ctx;
 };