Stop configuring install_command in tox.

It turns out that this is the the default value provided by tox:

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

So we can remove the line and simply use the default value.

Change-Id: Id3a4bc58d0322205456156808e482e9a137b3029
Co-Authored-By: tushargite96 <tushargite96@gmail.com>

tox.ini

@@ -6,8 +6,8 @@ ignore_basepython_conflict = True
 
 [testenv]
 basepython = python3
-install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 
 [testenv:releasenotes]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+       -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html