httpfile: remove size check failure backoff handling

httpfile: remove size check failure backoff handling

This backoff handling in HTTPFile is redundant for several reasons:

* We rely on the monitor worker anyways to inform us of unreachable hosts

* Monitor runs much faster nowadays, giving us a smaller window for
  out-of-date information about host reachability

* HTTPFile->size no longer connects to the sidechannel port,
  only HTTP, so we waste fewer syscalls on failure if we a
  host went down before the last monitor run.