Remove unused style helper (#3145)

app/helpers/style_helper.rb

-# frozen_string_literal: true
-
-module StyleHelper
-  def stylesheet_for_layout
-    if asset_exist? 'custom.css'
-      'custom'
-    else
-      'application'
-    end
-  end
-
-  def asset_exist?(path)
-    if Rails.configuration.assets.compile
-      Rails.application.precompiled_assets.include? path
-    else
-      Rails.application.assets_manifest.assets[path].present?
-    end
-  end
-end