Stop linking images to new tab

This fixes one of our [accessibility issues][1]. Specifically, it will
prevent images to link to new tabs without telling the user, which fails
[WCAG 2.1 success criterion 2.4.4 Link Purpose (in context)][2].

[1]: https://docs.google.com/spreadsheets/d/1s53eGMBj2hdTVLRitwi673R-_uERWh0kOxOp9QsOVg0
[2]: https://www.w3.org/TR/UNDERSTANDING-WCAG20/navigation-mechanisms-refs.html

Trello card: https://trello.com/c/vv0OR35g/630-march-update-tech-docs-template

lib/govuk_tech_docs/tech_docs_html_renderer.rb

@@ -20,7 +20,7 @@ def header(text, level)
     end
 
     def image(link, *args)
-      %(<a href="#{link}" target="_blank" rel="noopener noreferrer">#{super}</a>)
+      %(<a href="#{link}" rel="noopener noreferrer">#{super}</a>)
     end
 
     def table(header, body)