Remove PyPy from default tox runlist

PyPy has been failing locally for a month or two. On Ubuntu 14.04: 
"No module named _bsddb", on 12.04: "Not a .py file: 
/data/dev/work/ironic-python-agent/.tox/pypy/build/setuptools/
setuptools/cli-32.exe"

PyPy isn't enabled as a job in the gate yet (even non-voting). Until 
we sort the gate out and fix the above errors, we should remove it 
from the default tox runlist.

Change-Id: I34c0a72edebc296b79c6058e64ffb98f60979bcb