Remove conditional check on docker network plugin existence.

[nathanhammond]

This PR removes the check for existence of a network driver prior to attempting use.

Plugins are not activated automatically at Docker daemon startup. Rather, they are activated only lazily, or on-demand, when they are needed. https://docs.docker.com/engine/extend/plugin_api/#plugin-activation

As a consequence, if you write a custom network driver and attempt to use it for your Azure Pipelines Agent, the plugin has not been registered at that time.

You instead get the warning message and shunted back to the default.

This code was added in #3751 and this would be the first update since landing it.

---

Without this change, attempting to use a custom network plugin fails:

/usr/bin/docker info -f "{{range .Plugins.Network}}{{println .}}{{end}}"
bridge
host
ipvlan
macvlan
null
overlay
##[warning]Specified 'wireguard' driver not found!
/usr/bin/docker network create --label 8578ca vsts_network_174a1ae0516444449e9a26cc428af7c8
be79b766bbd1cfcd57483a66867f30be54191a7a784847d5d83c07a4a224938f