worker/upgradesteps: remove unnecessary machineId field
worker/upgradesteps: remove unnecessary machineId field

The machine id can be trivially retrieved from the tag, so there's no
need to store both.

perrito666/juju,howbazaar/juju,alesstimec/juju,perrito666/juju,macgreagoir/juju,marcmolla/juju,bz2/juju,howbazaar/juju,bac/juju,voidspace/juju,anastasiamac/juju,makyo/juju,gabriel-samfira/juju,reedobrien/juju,bac/juju,mikemccracken/juju,dooferlad/juju,kat-co/juju,gabriel-samfira/juju,dimitern/juju,makyo/juju,wwitzel3/juju,reedobrien/juju,fwereade/juju,perrito666/juju,mjs/juju,axw/juju,alesstimec/juju,voidspace/juju,axw/juju,frankban/juju,dimitern/juju,waigani/juju,axw/juju,ericsnowcurrently/juju,AdamIsrael/juju,mjs/juju,macgreagoir/juju,waigani/juju,bz2/juju,dimitern/juju,mikemccracken/juju,dooferlad/juju,howbazaar/juju,kat-co/juju,wwitzel3/juju,bogdanteleaga/juju,axw/juju,bogdanteleaga/juju,bogdanteleaga/juju,frankban/juju,ericsnowcurrently/juju,voidspace/juju,anastasiamac/juju,bogdanteleaga/juju,davecheney/juju,ericsnowcurrently/juju,ericsnowcurrently/juju,howbazaar/juju,davecheney/juju,mjs/juju,bac/juju,ericsnowcurrently/juju,kat-co/juju,gabriel-samfira/juju,AdamIsrael/juju,frankban/juju,macgreagoir/juju,mjs/juju,alesstimec/juju,bz2/juju,AdamIsrael/juju,dooferlad/juju,davecheney/juju,fwereade/juju,anastasiamac/juju,fwereade/juju,wwitzel3/juju,bac/juju,wwitzel3/juju,frankban/juju,marcmolla/juju,makyo/juju,AdamIsrael/juju,dooferlad/juju,kat-co/juju,voidspace/juju,dimitern/juju,marcmolla/juju,reedobrien/juju,mjs/juju,mikemccracken/juju,reedobrien/juju,waigani/juju,bz2/juju,waigani/juju,anastasiamac/juju,mjs/juju,kat-co/juju,mjs/juju,macgreagoir/juju,dooferlad/juju,gabriel-samfira/juju,frankban/juju,anastasiamac/juju,davecheney/juju,mikemccracken/juju,gabriel-samfira/juju,bac/juju,perrito666/juju,marcmolla/juju,marcmolla/juju,bz2/juju,alesstimec/juju,perrito666/juju,fwereade/juju,dimitern/juju,mikemccracken/juju,reedobrien/juju,howbazaar/juju,axw/juju,macgreagoir/juju,makyo/juju,alesstimec/juju