[FIXED JENKINS-18836][FIXED JENKINS-18879][FIXED JENKINS-19619] remove double call of freeupWindow(len); when using ssh-slaves 0.27+

- the more performant code path is only followed when using SSH Slaves 0.27+
- the double call causes the channel to get torn down
- thus excessive logging to stderr on the slave side of the connection will cause the connection to tear down
- removing the duplicate call resolves the issue