replace remaining vm_destroy call-sites with destroy_vm

replace remaining vm_destroy call-sites with destroy_vm

This function has been used in one place only into which we inlined its
functionality. Removing it avoids confusion between vm_destroy and vm_destroy.

The whole $importfn is executed in a lock_config_full.
As a consequence, for the inlined code:
1. lock_config is redundant
2. it is not possible that the VM has been started (check_running) in the
    meanwhile
Additionally, it is not possible that the "lock" property has been written into
the VM's config file (check_lock) in the meanwhile

Add warning after eval so that it does not go unnoticed if it ever comes into
action.

Signed-off-by: Dominic Jäger <d.jaeger@proxmox.com>