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: I0aafce297f071cfaea89c3c3882db92c9f6d6783
Co-Authored-By: tushargite96 <tushargite96@gmail.com>