Don't set DEVSTACK_GATE_CLEAN_LOGS

The variable was removed from devstack-gate as of:
Id236826a1f2669c92b20cf8f07ee604f2c12f06c

Change-Id: I9053d5edddd2fc11a7e6554563bc1c8c80081b29
Merge "Don't set DEVSTACK_GATE_CLEAN_LOGS"