Remove redundant link_updated code related to bookmarks

[solarissmoke]

The links table has a column called link_updated which used to be used to store the date the link last changed (using pingomatic to check for this).

The link update checking was removed from core some time ago, see #12437, [13744] (also #4230, #4231). However there's still some code in wp-admin/bookmark.php and wp-admin/bookmark-template.php which uses link_updated as well as the links_recently_updated_time option. I guess the column in the database can't just be dropped because of back compat. But the code that uses it in core looks like it can.

Actually there are quite a few related options lurking in the database:

'links_updated_date_format' => __('F j, Y g:i a'),
'links_recently_updated_prepend' => '<em>',
'links_recently_updated_append' => '</em>',
'links_recently_updated_time' => 120,

Patch attached, lots of red. This will probably break plugins that were using those options. A ​quick search says there are very few, although this isn't necessarily a good enough reason to drop them.