make request/response builders actually build

[gudvinr]

It's a follow-up for #4353. Sorry for the noise.

Didn't occur to me that request/response builders are actually not being used as builders.

Couple notes here:

1. CreateFeedFromSubscriptionDiscovery made useless builder. It is supposedly intended to be used with NewIconChecker but NewIconChecker makes its own builder.
   I removed builder in CreateFeedFromSubscriptionDiscovery because it wasn't used. If that's the one actually should've been used in first place, please let me know.

2. In theory builders should be immutable. I.e. if you do newBuilder := oldBuilder.Sumsin(...), state of oldBuilder is unchanged.

With that assumption in mind (this is just my guess) subscriptionFinder.findSubscriptionsFromWellKnownURLs called WithoutRedirects in a loop for each URL. That in theory should create ephemeral builder with redirects disabled but in fact it just disables redirects over and over for no reason.

To be able to use that technique one probably should actually make real builder that doesn't change global builder state. However, currently no builder in miniflux does that and it'd be quite tedious task for basically no gain.

I've decided to just move WithoutRedirects up so it's just explicitly does what it's already doing. If something else should be done insted, I'm all ears.

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[fguillot]

There is a regression in FindSubscriptions. Previously redirects were only disabled for well-known feed probes in findSubscriptionsFromWellKnownURLs. Now the WithoutRedirects() is applied to the shared RequestBuilder before FindSubscriptions(). This breaks common subscription flows with expected redirects.

[gudvinr]

Previously redirects were only disabled for well-known feed probes in findSubscriptionsFromWellKnownURLs.

As WithRedirects changes struct field by pointer, redirects were disabled for well-known probes and all probes after first WithRedirects. If I understand this code correctly.

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Went back to previous behaviour anyway. This indeed doesn't look like a good solution.