Merge christmas_tree_ornament and christmas_ornament

BUR #46015 has been approved by @evazion.

create alias christmas_tree_ornament -> christmas_ornaments

From what I can tell, these two refer to the decorations put up on a Christmas tree

This bulk update request is pending automatic rejection in 5 days.

This bulk update request has been rejected because it was not approved within 45 days.

The bulk update request #46015 (forum #377957) has been rejected by @DanbooruBot.

The bulk update request #46015 (forum #377957) has been approved by @evazion.