Remove pbr warnerrors in favor of sphinx check

This change removes the now unused "warnerrors" setting,
which is replaced by "warning-is-error" in sphinx
releases >= 1.5 [1].

[1] http://lists.openstack.org/pipermail/openstack-dev/
2017-March/113085.html

Change-Id: I3f558a5bbb5c536c26929e9e6d79628a7e377a9a