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: Iafcae9fb2c7ae1046bec6fc38d9f57a6bff7e088

tox.ini

@@ -1,11 +1,10 @@
 [tox]
-minversion = 1.6
+minversion = 3.2.0
 skipsdist = True
 envlist = linters
 
 [testenv]
 basepython = python3
-install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
 
 [testenv:bindep]