Remove minikube support for external machine drivers

[afbjorklund]

Once the legacy drivers are removed, there is no need for the external drivers anymore...

Then all drivers are builtin to minikube, and there is no external libmachine (or drivers).

• hyperkit => obsolete, deprecated - was using setuid root on the external driver

• kvm2 => changed to use dlopen instead, so that it can be compiled with minikube

• parallels => compiled in, deprecated - copyright and code added to minikube

• vmware => compiled in, deprecated - copyright and code added to minikube

No need to build anymore external programs, can avoid using "docker-machine-driver"

Eventually when libmachine is fully integrated, the RPC code can be removed altogether...

• Remove old references to docker machine #21619

https://github.com/minikube-machine/machine/tree/main/drivers

drivers/generic
drivers/hyperv
drivers/none
drivers/virtualbox