Remove duplicate lower() call.
Remove duplicate lower() call.

We already make sure all members of existing_labels and new_labels are
lowercase.

PriviPK/privipk-sync-engine,rmasters/inbox,EthanBlackburn/sync-engine,ErinCall/sync-engine,gale320/sync-engine,nylas/sync-engine,PriviPK/privipk-sync-engine,EthanBlackburn/sync-engine,ErinCall/sync-engine,wakermahmud/sync-engine,ErinCall/sync-engine,ErinCall/sync-engine,nylas/sync-engine,jobscore/sync-engine,EthanBlackburn/sync-engine,PriviPK/privipk-sync-engine,jobscore/sync-engine,EthanBlackburn/sync-engine,closeio/nylas,closeio/nylas,Eagles2F/sync-engine,EthanBlackburn/sync-engine,Eagles2F/sync-engine,Eagles2F/sync-engine,wakermahmud/sync-engine,Eagles2F/sync-engine,gale320/sync-engine,wakermahmud/sync-engine,nylas/sync-engine,rmasters/inbox,ErinCall/sync-engine,closeio/nylas,gale320/sync-engine,jobscore/sync-engine,PriviPK/privipk-sync-engine,Eagles2F/sync-engine,gale320/sync-engine,PriviPK/privipk-sync-engine,wakermahmud/sync-engine,jobscore/sync-engine,closeio/nylas,rmasters/inbox,gale320/sync-engine,nylas/sync-engine,rmasters/inbox,wakermahmud/sync-engine