Removed undefined method in install_env.py file

There was a call from install_venv.py file to post_process method
of InstallVenv class, but this method is not defined in this class
as result it was raising an error. Even when this error doesn't
affect dependencies installation, it is not allowing to display user
information.

Change-Id: I499c0399eb961588d7e1491e1481412ffdda38b8
Closes-Bug: #1275025