Sitemaps: option not disabling when another Sitemaps plugin is activated.

#### Steps to reproduce the issue

1. Enable sitemap on Jetpack site.
2. Install and activate an SEO plugin (Yoast) and enable sitemaps in the plugin.

#### What I expected

When an SEO plugin is activated, all of the SEO settings (including generation of sitemaps) should be disabled/not active.

#### What happened instead

Jetpack is still showing the sitemap setting (as well as the sitemap itself). The Yoast (or other detected SEO plugin) sitemap should be the only sitemaps available to the crawler, and reflected in the plugin-created robots.txt (which should be managed by the plugin when active).

SEO settings still enabled in WP-Admin:
http://cld.wthms.co/TxW3wX

SEO settings partially enabled in Calypso:
http://cld.wthms.co/SOdCNu

Reproduce this with Jetpack sitemaps enabled, then activate Yoast and enable its sitemaps. Trace the Jetpack SEO settings and sitemap or robots.txt behavior described in the issue. Done means the competing SEO plugin is the sole sitemap and robots.txt provider, while Jetpack SEO settings are disabled in WP-Admin and Calypso.

Written by the indexing model from the issue text.