decom the decom script

https://wikitech.wikimedia.org/wiki/Server_Lifecycle
says:

"Inclusion in decommissioning.pp removes the host
from puppet's storedconfig, but with a delay of
several hours to a day, and not always reliably,
so we don't rely on that mechanism any more."

that is why we have stopped using it since quite a while

instead we just use puppetstoredconfigclean.rb  manually
on the puppetmaster to decom

so this is not in sync at all anyways, and we should just
remove it for being called unreliable, having that delay
and not containing valid info anymore.

to check what has _not_ been decom'ed yet in Tampa,
check dsh groups instead

Change-Id: I76c59c5f23647c756ad7a5d06ff2b6ed74804b37