Merge pull request #671 from citibeth/efischer/160327-RemoveRedundantLibXML2

Removed redundant package py-libxml2.  Use libxml2 instead.

var/spack/repos/builtin/packages/py-libxml2/package.py

-from spack import *
-
-class PyLibxml2(Package):
-    """A Python wrapper around libxml2."""
-    homepage = "https://xmlsoft.org/python.html"
-    url      = "ftp://xmlsoft.org/libxml2/python/libxml2-python-2.6.21.tar.gz"
-
-    version('2.6.21', '229dd2b3d110a77defeeaa73af83f7f3')
-
-    extends('python')
-    depends_on('libxml2')
-    depends_on('libxslt')
-
-    def install(self, spec, prefix):
-        python('setup.py', 'install', '--prefix=%s' % prefix)