Remove unnecessary `coffee-rails`

Since we no longer use coffeescript in the generated files by
4838c171, this is no longer necessary.

Gemfile

@@ -12,7 +12,6 @@ gem "rake", ">= 11.1"
 gem "capybara", ">= 2.15"
 
 gem "rack-cache", "~> 1.2"
-gem "coffee-rails"
 gem "sass-rails"
 gem "turbolinks", "~> 5"
 gem "webpacker", github: "rails/webpacker", require: ENV["SKIP_REQUIRE_WEBPACKER"] != "true"

Gemfile.lock

@@ -193,9 +193,6 @@ GEM
     chromedriver-helper (2.0.0)
       archive-zip (~> 0.10)
       nokogiri (~> 1.8)
-    coffee-rails (4.2.2)
-      coffee-script (>= 2.2.0)
-      railties (>= 4.0.0)
     coffee-script (2.4.1)
       coffee-script-source
       execjs
@@ -529,7 +526,6 @@ DEPENDENCIES
   byebug
   capybara (>= 2.15)
   chromedriver-helper
-  coffee-rails
   connection_pool
   dalli
   delayed_job