Fix tox docs target

When the upper constraint usage moved to install_command in tox.ini, it
was not removed from docs target's deps list, where it's now not
necessary, and even could cause issues when overriding
TOX_CONSTRAINTS_FILE.

This patch replaces the old UPPER_CONSTRAINTS_FILE to the "new"
TOX_CONSTRAINTS_FILE in the install_command, too.

Change-Id: I9c27caa869d72e06e84bcf84ceedb586ed3bc8e9

tox.ini

@@ -10,7 +10,7 @@ allowlist_externals =
   rm
   env
   make
-install_command = python -I -m pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
+install_command = python -I -m pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
   VIRTUAL_ENV={envdir}
   LANGUAGE=en_US
@@ -194,7 +194,6 @@ description =
 # Note that we don't use {[testenv]deps} for deps here because we don't want
 # to install (test-)requirements.txt for docs.
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
   -r{toxinidir}/doc/requirements.txt
 extras =