Fix duplicate `title` if `jekyll-seo-tag` not in users's plugins (#1040)

In case 'just-the-docs' is used as a theme, and a user does not
eplicitly include 'jekyll-seo-tag' inside the plugins list in
_config.yml, two 'title' elements were rendered. Since
jekyll-seo-tag is always available, because it's set in the plugins
list of the theme itself, the 'seo' tag will aways work, making
the seperate title and description elements obsolete.

Fixes #1008