This allows the URL to the other wiki's rest.php to be configured
exactly, rather than assuming that it has the same wgScriptPath as the
current wiki. This is necessary to make this feature work on PatchDemo,
where wgScriptPath looks like '/123abc456/w'.
$wgVectorSearchHost is removed, since nothing uses it except PatchDemo
(where it's broken) and development setups.
Bug: T319494
Change-Id: Ife042f4f683d366a31a642723746d4aa80774c03