Remove unecessary peer dependencies on gatsby-source-wordpress

[laurent-le-graverend]

gatsby-source-wordpress requires plugins like gatsby-plugin-image and gatsby-plugin-sharp.

https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-source-wordpress/package.json#L77-L79

However, these are unnecessary as it is optional to use them with this plugin.
It would be ideal not to make them mandatory to keep dependencies as light as possible.