Clean up confusing code in LfsListService

We had to pass HEAD to get all the references but
since https://gitlab.com/gitlab-org/gitaly/issues/2247 is fixed, we don't
need it anymore

app/services/projects/lfs_pointers/lfs_list_service.rb

@@ -4,14 +4,12 @@
 module Projects
   module LfsPointers
     class LfsListService < BaseService
-      REV = 'HEAD'
-
       # Retrieve all lfs blob pointers and returns a hash
       # with the structure { lfs_file_oid => lfs_file_size }
       def execute
         return {} unless project&.lfs_enabled?
 
-        Gitlab::Git::LfsChanges.new(project.repository, REV)
+        Gitlab::Git::LfsChanges.new(project.repository)
                                .all_pointers
                                .map! { |blob| [blob.lfs_oid, blob.lfs_size] }
                                .to_h

lib/gitlab/git/lfs_changes.rb

@@ -3,7 +3,7 @@
 module Gitlab
   module Git
     class LfsChanges
-      def initialize(repository, newrev)
+      def initialize(repository, newrev = nil)
         @repository = repository
         @newrev = newrev
       end