Remove all permission checking from Reference filters

lib/gitlab/markdown/cross_project_reference.rb

lib/gitlab/markdown/user_reference_filter.rb

...lib/gitlab/markdown/commit_range_reference_filter_spec.rb

spec/lib/gitlab/markdown/commit_reference_filter_spec.rb

spec/lib/gitlab/markdown/cross_project_reference_spec.rb

spec/lib/gitlab/markdown/issue_reference_filter_spec.rb

...ib/gitlab/markdown/merge_request_reference_filter_spec.rb

spec/lib/gitlab/markdown/snippet_reference_filter_spec.rb

spec/lib/gitlab/markdown/user_reference_filter_spec.rb

spec/support/filter_spec_helper.rb