Remove mock in lower-constraints.txt

This is a follow up patch for I4d45ae17f6f84f945f5dd049a929216ce6b6b58e.

The 'mock' line has already been removed in test-requirements.txt.
So the 'mock' line is removed in lower-constraints.txt.

Change-Id: Ic215d48477cfb7441e3bddba969258157468713c
Signed-off-by: Takashi Natsume <[email protected]>

Author: takanattie

lower-constraints.txt

@@ -38,7 +38,6 @@ kombu==4.0.0
 linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
-mock==2.0.0
 monotonic==0.6
 msgpack-python==0.4.0
 munch==2.1.0