fix: compute max #file chunks from raw transaction size limit and file append chunk size

[david-bakin-sl]

There are configuration items for SEND_RAW_TRANSACTION_SIZE_LIMIT, FILE_APPEND_MAX_CHUNKS, and FILE_APPEND_CHUNK_SIZE. FILE_APPEND_MAX_CHUNKS can (and should) be computed from the other two. So it can be eliminated as a config variable, and maintainer doesn't have to remember to update it in sync with changes to SEND_RAW_TRANSACTION_SIZE_LIMIT.