bug#55275: 29.0.50; Gnus' nnvirtual-retrieve-headers shouldn't force usage of the cache

[Eric Abrahamsen]

In ./lisp/gnus/nnvirtual.el, the `nnvirtual-retrieve-headers' deffoo
let-binds `gnus-use-cache' to t when it uses `gnus-retrieve-headers' on
its various constituent groups. For users not using the cache, this
will result in an error.

I can put an additional check in gnus-cache.el when we call
`gnus-cache-retrieve-headers', but it would be nice to know why this
necessary in the first place. The code comment on
`nnvirtual-retrieve-headers' says:

;; FIX FIX FIX we want to check the cache!
;; This is probably evil if people have set
;; gnus-use-cache to nil themselves, but I
;; have no way of finding the true value of it.

All the above is true, except I don't understand why we want to check
the cache. This code is ancient and I'm only medium-hopeful Lars
remembers the reasoning here, but on the face of it I don't know why we
need to force a check of the cache. It seems like we could simply remove
that, and the cache will still be checked iff the user has set
`gnus-use-cache' to t.