setup.cfg: Explicitly set [build_sphinx] builder

An recent change in pbr changed the default behavior of pbr's variants
of the 'sphinx_build' setuptools plugin [1]. Previously, pbr defaulted
to building both HTML and man page output, but it now builds only the
former. nova happens to be one of the rare projects that _does_ care
about man pages so mitigate the impact of this change by explicitly
setting the 'builder' configuration option to build HTML and man page
output.

While we're at it, the 'build_apiguide' section is removed as this
doesn't actually do anything and is a remnant of a broken cookiecutter
file [2].

[1] https://review.openstack.org/#/c/456157/
[2] https://review.openstack.org/#/c/448164/

Change-Id: I79efedd44b11c9832ae8cb36e54c2567f85fcf50

setup.cfg

@@ -93,16 +93,12 @@ nova.scheduler.driver =
     fake_scheduler = nova.tests.unit.scheduler.fakes:FakeScheduler
 
 [build_sphinx]
-all_files = 1
+builder = html man
+all-files = 1
 build-dir = doc/build
 source-dir = doc/source
 warning-is-error = 1
 
-[build_apiguide]
-all_files = 1
-build-dir = api-guide/build
-source-dir = api-guide/source
-
 [egg_info]
 tag_build =
 tag_date = 0