Use default max packet size for SFTP

[busser]

This pull request changes nmon2influxdb so it uses the default MaxPacket size from from pkg/sftp.

A change to pkg/sftp (this commit) made in January broke nmon2influxdb's capacity to fetch remote files.

The reason is that the MaxPacket size used by nmon2influxdb, 64000 bytes, is too high. Changing it to any value less than or equal to 32768 bytes (=32KiB) would fix the problem.

I think a cleaner solution is to use the default package size from pkg/sftp, which is 32768 bytes. The MaxPacket parameter is no longer required when creating a new SFTP client.

[adejoux]

Thanks