Fix #5045: Exception on uploads when SauceNAO is the referrer URL.

Bug: We assumed the referer URL was from the same site as the target
URL. We tried to call methods on the referer only supported by the
target URL.

Fix: Ignore the referer URL when it's from a different site than the
target URL.

app/logical/sources/strategies/nico_seiga.rb

@@ -36,7 +36,6 @@ module Sources
       end
 
       def page_url
-        # XXX what if referer isn't from NicoSeiga?
         parsed_referer&.page_url || parsed_url.page_url
       end