Remove bin/generate-post-meta-image (#569)

Why:
* Images for SEO are now generated via JS, there's no need for this
  script

How:
* Deleting `bin/generate-post-meta-image`, `Gemfile`, `Gemfile.lock`
* Removing `Ruby` from  `bin/setup` and `.tool-versions`

Author: DavideSilva

.tool-versions

@@ -1,2 +1 @@
 nodejs 19.6.0
-ruby 2.6.2

Gemfile

@@ -11,10 +11,3 @@ asdf reshim nodejs
 
 # Install JavaScript dependencies
 yarn install
-
-# Install or update Bundler globally
-gem install bundler --conservative
-asdf reshim ruby
-
-# Install Ruby dependencies
-bundler install