v2v: remove extra nbdkit bit from documentation (RHBZ#1723305)

Since there is no more need to build nbdkit from sources, then there is
no need to set $PATH with a custom build of nbdkit.

Followup of commit 0704d8eb0b.

v2v/virt-v2v-input-vmware.pod

@@ -293,7 +293,6 @@ To import a particular guest from vCenter server or ESXi hypervisor,
 use a command like the following, substituting the URI, guest name and
 SSL thumbprint:
 
- $ export PATH=/path/to/nbdkit-1.1.x:$PATH
  $ virt-v2v \
      -ic 'vpx://root@vcenter.example.com/Datacenter/esxi?no_verify=1' \
      -it vddk \