Stop configuring install_command in tox.

Currently, we are overriding 'install_command' to use 'pip'. This is
considered poor behavior and 'python -m pip' should be used instead:

https://snarky.ca/why-you-should-use-python-m-pip/

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: Ideb8c351a88f55592d58e6c42709db76de50c82b

tox.ini

@@ -5,7 +5,6 @@ envlist = pep8,py27,py37,tht
 
 [testenv]
 usedevelop = True
-install_command = {envpython} -m pip install {opts} {packages}
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
        -r{toxinidir}/requirements.txt