SFTP: Replace incorrect comment about filesize. There is no 4 GiB limit.

* There is no 2 or 4 GiB limit. It has been removed by 2c80ac8acad6d4b0c486d4e5db63ac6e27b3e4ea (which was followed by 0f5b3ea41613545869dc2546b691e4e686796591).
* The BigInteger comment is suboptimal. As we have seen, we can use floats for the time being.

I am not sure there are any problems with the actual statements below, but here is an alternative version which may or may not have more desirable properties: `$attr['size'] = hexdec(bin2hex($this->_string_shift($response, 8)));`