Remove graphiz from test-requirements.txt

With the split of kolla and kolla-ansible, it seems that this
dependency was carried over as it is used when generating
graphs for build sequences.

This patch removes it as the build process is not part of this
project anymore.

Change-Id: I7fd59fe703c2f06c003c8acda15f2a06d3fd7f3f

test-requirements.txt

@@ -7,7 +7,6 @@ beautifulsoup4 # MIT
 coverage>=4.0 # Apache-2.0
 doc8 # Apache-2.0
 extras # MIT
-graphviz!=0.5.0,>=0.4.0 # MIT License
 hacking>=0.10.0
 openstackdocstheme>=1.5.0 # Apache-2.0
 oslo.log>=3.22.0 # Apache-2.0