Make `warn` deprecation note a comment instead

This turned out to be quite noisy, and we already have a dedicated issue
to track bulk insert related deprecations.

https://gitlab.com/gitlab-org/gitlab/issues/207989

app/models/merge_request_diff_commit.rb

@@ -10,8 +10,9 @@ class MergeRequestDiffCommit < ApplicationRecord
   sha_attribute :sha
   alias_attribute :id, :sha
 
+  # Deprecated; use `bulk_insert!` from `BulkInsertSafe` mixin instead.
+  # cf. https://gitlab.com/gitlab-org/gitlab/issues/207989 for progress
   def self.create_bulk(merge_request_diff_id, commits)
-    warn 'Deprecated; use `bulk_insert` from `BulkInsertSafe` mixin instead'
     rows = commits.map.with_index do |commit, index|
       # See #parent_ids.
       commit_hash = commit.to_hash.except(:parent_ids)