Drop explicit dependency on python-subunit

It is no longer directly used by any test code in this repository since
we switched to stestr[1].

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.

[1] 22e10bc884

Change-Id: I14c166abd63ded5c63275e6e536286d62fda4f12

doc/requirements.txt

@@ -1,7 +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
 setuptools!=24.0.0,!=34.0.0,!=34.0.1,!=34.0.2,!=34.0.3,!=34.1.0,!=34.1.1,!=34.2.0,!=34.3.0,!=34.3.1,!=34.3.2,!=36.2.0,>=21.0.0;python_version>='3.12' # PSF/ZPL
 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.
-
 cliff!=2.9.0,>=2.8.0 # Apache-2.0
 keystoneauth1>=3.18.0 # Apache-2.0
 python-neutronclient>=6.7.0 # Apache-2.0

test-requirements.txt

@@ -1,7 +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.
-
 hacking>=6.1.0,<6.2.0 # Apache-2.0
 requests-mock>=1.2.0 # Apache-2.0
 coverage!=4.4,>=4.0 # Apache-2.0
@@ -10,7 +6,6 @@ Pygments>=2.2.0 # BSD license
 bandit!=1.6.0,>=1.1.0 # Apache-2.0
 flake8-import-order>=0.18.0,<0.19.0 # LGPLv3
 pylint>=2.5.3 # GPLv2
-python-subunit>=1.0.0 # Apache-2.0/BSD
 oslotest>=3.2.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD