libxl: do not attempt to write "shutdown" dm-command

libxl: do not attempt to write "shutdown" dm-command

libxl_domain_destroy writes the command "shutdown" to the xenstore
node with which it communicates with qemu.  However:
 - qemu does not understand this command and ignores it (printing a
   message)
 - libxl doesn't wait for the answer and immediately pauses the domain
   anyway
 - destroy is the ungraceful (force) operation and should not
   negotiate with qemu anyway
 - even in the graceful shutdown case, there would actually be nothing
   that qemu needs to do.

Under some circumstances, this entry in xenstore will survive the
domain's death, ie be leaked.

So remove the erroneous code.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>