Drop explicit dependency on python-subunit

It is no longer directly used by any test code in this repository. It
is now installed as a dependency of stestr.

Also remove the note about behavior of old pip, because recent pip can
resolve dependencies without requiring a specific order.

Change-Id: Iad3714802c9afb63df3360c4e400ae6dcb2bc87b

Author: kajinamit

doc/requirements.txt

@@ -1,6 +1,3 @@
-# The order of packages is significant, because pip processes them in the order
-# of appearance. Changing the order has an impact on the overall integration
-# process, which may cause wedges in the gate later.
 sphinx>=2.0.0,!=2.1.0  # BSD
 sphinxcontrib-svg2pdfconverter>=0.1.0 # BSD
 openstackdocstheme>=2.2.1 # Apache-2.0

requirements.txt

@@ -2,10 +2,6 @@
 # date but we do not test them so no guarantee of having them all correct. If
 # you find any incorrect lower bounds, let us know or propose a fix.
 
-# The order of packages is significant, because pip processes them in the order
-# of appearance. Changing the order has an impact on the overall integration
-# process, which may cause wedges in the gate later.
-
 keystoneauth1>=3.14.0 # Apache-2.0
 netaddr>=0.7.18 # BSD
 neutron-lib>=3.8.0 # Apache-2.0

test-requirements.txt

@@ -1,13 +1,8 @@
-# The order of packages is significant, because pip processes them in the order
-# of appearance. Changing the order has an impact on the overall integration
-# process, which may cause wedges in the gate later.
-
 hacking>=6.1.0,<6.2.0 # Apache-2.0
 
 bandit!=1.6.0,>=1.1.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
 flake8-import-order>=0.18.0,<0.19.0 # LGPLv3
-python-subunit>=1.0.0 # Apache-2.0/BSD
 oslotest>=3.2.0 # Apache-2.0
 stestr>=1.0.0 # Apache-2.0
 pylint>=2.6.0 # GPLv2