Remove the mdl gem as it's no longer in use

Gemfile

@@ -367,7 +367,6 @@ group :development, :test do
   gem 'haml_lint', '~> 0.31.0', require: false
   gem 'simplecov', '~> 0.16.1', require: false
   gem 'bundler-audit', '~> 0.5.0', require: false
-  gem 'mdl', '~> 0.5.0', require: false
   gem 'benchmark-ips', '~> 2.3.0', require: false

Gemfile.lock

@@ -475,7 +475,6 @@ GEM
     kgio (2.11.2)
     knapsack (1.17.0)
       rake
-    kramdown (1.17.0)
     kubeclient (4.2.2)
       http (~> 3.0)
       recursive-open-struct (~> 1.0, >= 1.0.4)
@@ -511,10 +510,6 @@ GEM
     mail_room (0.9.1)
     marcel (0.3.3)
       mimemagic (~> 0.3.2)
-    mdl (0.5.0)
-      kramdown (~> 1.12, >= 1.12.0)
-      mixlib-cli (~> 1.7, >= 1.7.0)
-      mixlib-config (~> 2.2, >= 2.2.1)
     memoist (0.16.0)
     memoizable (0.4.2)
       thread_safe (~> 0.3, >= 0.3.1)
@@ -528,9 +523,6 @@ GEM
     mini_mime (1.0.1)
     mini_portile2 (2.4.0)
     minitest (5.11.3)
-    mixlib-cli (1.7.0)
-    mixlib-config (2.2.18)
-      tomlrb
     msgpack (1.3.0)
     multi_json (1.13.1)
     multi_xml (0.6.0)
@@ -951,7 +943,6 @@ GEM
       parslet (~> 1.8.0)
     toml-rb (1.0.0)
       citrus (~> 3.0, > 3.0)
-    tomlrb (1.2.8)
     truncato (0.7.11)
       htmlentities (~> 4.3.1)
       nokogiri (>= 1.7.0, <= 2.0)
@@ -1146,7 +1137,6 @@ DEPENDENCIES
   lograge (~> 0.5)
   loofah (~> 2.2)
   mail_room (~> 0.9.1)
-  mdl (~> 0.5.0)
   memory_profiler (~> 0.9)
   method_source (~> 0.8)
   mimemagic (~> 0.3.2)