Remove pwb installation from setup.py

setup.py install is run by pip or directly by the user to install
a package, typically into a system wide location.  As this is a
system wide operation, it should not be followed by a per-user
configuration task like calling generate_user_files.py,
especially as it requires user interaction.

As a installed library, pywikibot should avoid requiring user
configuration files.

Change-Id: Ibcab9f783cbe8299f104c690466be32e22aa4c1a
Merge "Remove pwb installation from setup.py"