Avoid precompiling assets unnecessary

We had two calls to precompile assets, one for all specs and then a
second one just for system specs. The one for system specs is better
because it checks for the webpack dev server before compiling assets. So
let's use just that one.

spec/spec_helper.rb

@@ -4,12 +4,6 @@ require 'base_spec_helper'
 require 'database_cleaner'
 
 RSpec.configure do |config|
-  # Precompile Webpacker assets (once) when starting the suite. The default setup can result
-  # in the assets getting compiled many times throughout the build, slowing it down.
-  config.before :suite do
-    Webpacker.compile
-  end
-
   # Fix encoding issue in Rails 5.0; allows passing empty arrays or hashes as params.
   config.before(:each, type: :controller) { @request.env["CONTENT_TYPE"] = 'application/json' }
 end