Revert r47388 / 8d9243cf3: Use Title::getLocalURL() for rel=canonical links

Using relative links for <link rel=canonical> is a missed chance to signal the
preferred hostname and protocol for accessing our sites, which invites Google
(and other search engines) to guess, or to treat the request URL's hostname as
canonical. Compared to that, the number of bytes saved by using a relative link
is insubstantial.

Change-Id: I483b4425c1c14211e67bbb6e7ae077b7faa4ee25
(cherry picked from commit 6c3b7aa75fc700e85418761b499d73ec6acce256)