Remove `current_user` context from markdown and gfm helpers

These helpers are no longer dependent on the current user state. Hooray!

app/helpers/gitlab_markdown_helper.rb

@@ -46,7 +46,6 @@ module GitlabMarkdownHelper
 
   def markdown(text, context = {})
     context.merge!(
-      current_user: current_user,
       path:         @path,
       project:      @project,
       project_wiki: @project_wiki,
@@ -60,7 +59,6 @@ module GitlabMarkdownHelper
   # with a custom pipeline depending on the content being rendered
   def gfm(text, options = {})
     options.merge!(
-      current_user: current_user,
       path:         @path,
       project:      @project,
       project_wiki: @project_wiki,

lib/gitlab/markdown.rb

@@ -85,7 +85,6 @@ module Gitlab
         no_header_anchors: options[:no_header_anchors],
 
         # ReferenceFilter
-        current_user:    options[:current_user],
         only_path:       options[:reference_only_path],
         project:         options[:project],