SSH: Do not tie key exchange methods to specific host key algorithms.

Contrary to what I believed, all key exchange methods are compatible
with all types of host key (at least for everything we support).

Task-number: QTCREATORBUG-14621
Change-Id: Iacae3d36170b9841cceb3705780a64aeb15e10b9
Reviewed-by: Benjamin Zeller <benjamin.zeller@canonical.com>
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>