Remove old links_recently_updated_* options

[nacin]

	'links_recently_updated_prepend' => '<em>',
	'links_recently_updated_append' => '</em>',
	'links_recently_updated_time' => 120,

[nacin]

In 27916:

Remove old links_recently_updated_* DB options that never had a UI.

fixes #27649.

[nacin]

In 27917:

Remove links_recently_updated_time (hardening).

Merges some of [27916] to the 3.8 branch.

props Tom Adams.
see #27649.

[nacin]

In 27919:

Remove links_recently_updated_time (hardening).

Merges [27917] from the 3.8 branch to 3.7 branch.

see #27649.