Remove "Discourage search engine indexing" from being set on the Gatsby site pages.

[crock]

When you use this plugin with a headless wordpress instance, you don't want the WP instance to be indexed by search engines, so you tick checkbox in the WP ACP that says "Discourage search engine indexing". However, the problem comes when this Gatsby plugin carries over that setting to the Gatsby site, which should be indexed.

@ashhitch Is this something you would be open to being fixed if I open a PR for it? Should it be a configurable option in the gatsby-config?