Drop mock from lower-constraints.txt

The mock is not needed for py36 and later.

Change-Id: I24afe1f1255ac47dfba300946149732f70f5f399

Author: wangzihao

lower-constraints.txt

@@ -15,7 +15,6 @@ coverage==4.0 # Apache-2.0
 python-subunit==0.0.18 # Apache-2.0/BSD
 oslotest==1.10.0 # Apache-2.0
 stestr==2.0 # Apache-2.0
-mock==2.0 # BSD
 python-openstackclient==3.14 # Apache-2.0
 
 # doc/requirements.txt