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

setup.cfg

@@ -31,13 +31,11 @@ fixtures =
   mock>=2.0 # BSD
   fixtures>=3.0.0 # Apache-2.0/BSD
 
-[pbr]
-warnerrors = true
-
 [build_sphinx]
 source-dir = doc/source
 build-dir = doc/build
 all_files = 1
+warning-is-error = 1
 
 [upload_sphinx]
 upload-dir = doc/build/html