Remove need to configure default-virtual-host at subsystem level

There is currently needed host to which the deployments should be deployed by default. This is found out from default-virtual-host at undertow subsystem level. This value actually duplicates setting from the value set in default-host attribute of default server, as such it should be removed and thus config simplified by counting the needed value default server's default-host value.

Summing it up:
current behavior:

• default server value is determined by: /subsystem=undertow[default-server] attribute
• default virtual host value is determined by: /subsystem=undertow[default-virtual-host] attribute

proposed behavior:

• default server value is determined by: /subsystem=undertow[default-server] attribute (no change here)
• default virtual host value is determined by: /subsystem=undertow/server=VALUE_DETERMINED_IN_PREVIOUS_STEP[default-host] attribute
• deprecate /subsystem=undertow[default-virtual-host] attribute and remove it completely in some of upcoming versions