Revert "Add plantuml gem (#3005)"

This reverts commit 83b16e7ebd.

docs/Gemfile

@@ -2,7 +2,6 @@ source "https://rubygems.org"
 
 group :jekyll_plugins do
   gem "github-pages"
-  gem "jekyll-plantuml"
 end
 
 group :development do

docs/Gemfile.lock

@@ -130,8 +130,6 @@ GEM
     jekyll-optional-front-matter (0.3.2)
       jekyll (>= 3.0, < 5.0)
     jekyll-paginate (1.1.0)
-    jekyll-plantuml (1.4.2)
-      jekyll (> 2.0)
     jekyll-readme-index (0.3.0)
       jekyll (>= 3.0, < 5.0)
     jekyll-redirect-from (0.16.0)
@@ -263,7 +261,6 @@ PLATFORMS
 
 DEPENDENCIES
   github-pages
-  jekyll-plantuml
   webrick
 
 BUNDLED WITH

docs/_config.yml

@@ -10,6 +10,3 @@ aux_links:
     - https://core-design-history.herokuapp.com
   "GitHub":
     - https://github.com/communitiesuk/submit-social-housing-lettings-and-sales-data
-gems: ["jekyll-plantuml"]
-plugins:
-  - jekyll-diagrams