drop mock from lower-constraints

The mock third party library was needed for mock support in py2
runtimes. Since we now only support py36 and later, we don't need it
in lower-constraints.

These changes will help us to drop `mock` from openstack/requirements

Change-Id: I41603a01231477cad7af9c4ed14ba88a32063ca5

lower-constraints.txt

@@ -45,7 +45,6 @@ logutils==0.3.5
 Mako==1.0.7
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==1.4
 mox3==0.25.0
 msgpack==0.5.6