doc: fix build with sphinx 2.x

app.info() was deprecated on Jan 4, 2017 (sphinx-doc/sphinx#3267),
and removed as of Sphinx 2.0.0. This commit removes the usage of
app.info().

PR-URL: https://github.com/libuv/libuv/pull/2265
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

docs/src/sphinx-plugins/manpage.py

@@ -39,7 +39,6 @@ def man_role(name, rawtext, text, lineno, inliner, options={}, content=[]):
 
 
 def setup(app):
-    app.info('Initializing manpage plugin')
     app.add_role('man', man_role)
     app.add_config_value('man_url_regex', None, 'env')
     return