Remove redundant TitleIsAlwaysKnown() hook

* When a flow page is added with header/post, a flow revision will be created
for the page, so we can fall back to whatever mediawiki's mechanism for redlink

* Reduce some redundant queries. In a board page, if a post creator's user/user talk
page is flow enabled and has no content, it will keep firing a db query due to cache miss,
this is bad for a page with a lot of posts

Change-Id: I64b4c991c5d6c62f5cd24bece2670e07eda31867
Merge "Remove redundant TitleIsAlwaysKnown() hook"