[DRE-maint] ruby-net-sftp is marked for autoremoval from testing

[Debian testing autoremoval watch]

ruby-net-sftp 1:2.1.2-3 is marked for autoremoval from testing on 2017-09-03

It is affected by these RC bugs:
869174: ruby-net-sftp: FTBFS: ERROR: Test "ruby2.3" failed:   => 142: assert_raises(ArgumentError) { sftp.download("/path/to/remote", StringIO.new, :recursive => true) }