virtio-serial: Remove superfluous serial-* words

These likely were a blind copy-n-paste from hvterm.fs, but they
simply do not make any sense in virtio-serial.fs. The hvterm.fs is
always included from OF.fs, so the serial-* words are globally there.
virtio-serial.fs is only used within the virtio-serial device tree
nodes, to adding the serial-* words there is just superfluous.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>