Remove Sajax

Calls for the removal of Sajax go back to (at least) October of 2012, when it
was already possible to describe the library as "ancient".[0] The work to mark
the library as deprecated[1] and to update its callers in Core[2] was completed
by Alex Monk two years ago. Usage in extensions was removed long ago in most
cases, and the last holdout -- SecurePoll -- was migrated by Frances Hocutt
last month. There is a long tail of usage in the MediaWiki namespace, but it
should not block this patch.

[0]: http://thread.gmane.org/gmane.science.linguistics.wikipedia.technical/64380
[1]: I4a0af8986
[2]: I52f01e4e7
[3]: I1e4f3107e

Bug: T55120
Change-Id: I22b69113a67c0b3a0979ff6ba1854ee8e95c1b88