Channel.size() is costly, mutually exclusive, and on the critical path

stefania_alborghetti mentioned this already on another ticket, but I have lost track of exactly where. While benchmarking it became apparent this was a noticeable bottleneck for small in-memory workloads with few files, especially with RF=1. We should probably fix this soon, since it is trivial to do so, and the call is only to impose an assertion that our requested length is less than the file size. It isn't possible to safely memoize a value anywhere we can guarantee to be able to safely refer to it without some refactoring, so I suggest simply removing the assertion for now.