76deb30312 changed this to not just be the URL, but rather a
prefixed hash of the URL, but failed to update this location which
wrote to it. This meant that this pre-population step was writing to
the wrong keys in the durable cache, and thus ineffective.
Then, da33b72848 switched the cache to be in-memory, making this
write to the wrong keys in an in-process memory store. There is no
way to pre-fill this sort of cache, except at server start-up.
Finally, and most fundamentally, 8c0c9ca7a4 then disabled
`inline_url_embed_preview` by default, making the code entirely moot.
Remove the triply-unnecessary code.
ede4a88b49