Consider dropping `MAX_CONTENT_LENGTH` checks

[spacebear21]

since body is already allocated, and presumably does not exceed the app's limits by the time this code is run, maybe it's better to just remove MAX_CONTENT_LENGTH altogether?

Originally posted by @nothingmuch in #745 (comment)

[nothingmuch]

also this #745 (comment)

[spacebear21]

remaining TODOs per @nothingmuch :

1. (DONE) making the receiver's request body handling stricter
2. the same change to the sender
3. enforcing limits on payjoin-cli receiver, which currently uses body.collect()
4. same thing for payjoin-cli sender, which uses &response.bytes() (from reqwest)