zuul: python-statsd has been packaged

I had a hack in zuul::init which edited the dependency file in Zuul
source code to remove the 'statsd' dependency.  This is no more needed
since the python module has been packaged months ago.

Moreover, the file tools/pip-requires does not exist anymore in upstream
code.

Change-Id: I7b088775a6a08d75e5ac5c0639ec0994be192c99