Stop defualting to USER from env in arcanist
Stop defualting to USER from env in arcanist

Summary: This once made sense but hasn't been a reasonable default for a long time, get rid of it.

Test Plan: Ran "arc list".

Reviewers: llorca, toulouse

CC:

Differential Revision: 496

Symplicity/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,jra3/arcanist,aik099/arcanist,hach-que/arcanist,aik099/arcanist,wharris/arcanist,jra3/arcanist,phacility/arcanist,wxstars/arcanist,folsom-labs/arcanist,haskell-infra/arcanist,eSpark/arcanist,memsql/arcanist,Symplicity/arcanist,wharris/arcanist,PageUpPeopleOrg/arcanist-patches,codelabs-ch/arcanist,wikimedia/phabricator-arcanist,chundongwang/arcanist,haskell-infra/arcanist,zhihu/arcanist,Symplicity/arcanist,codelabs-ch/arcanist,julkiewicz/arcanist,zhihu/arcanist,aik099/arcanist,coursera/arcanist,codelabs-ch/arcanist,Khan/arcanist,chundongwang/arcanist,sneakpeeq/arcanist,kevinseelbach/arcanist,KieranHayes/arcanist,hach-que/arcanist,eSpark/arcanist,aik099/arcanist,r4nt/arcanist,wikimedia/phabricator-arcanist,Khan/arcanist,wharris/arcanist,calblueprint/arcanist,kevinseelbach/arcanist,chundongwang/arcanist,Khan/arcanist,PageUpPeopleOrg/arcanist-patches,kevinseelbach/arcanist,hach-que/arcanist,r4nt/arcanist,uber/arcanist,uber/arcanist,coursera/arcanist,uber/arcanist,wxstars/arcanist,julkiewicz/arcanist,kevinseelbach/arcanist,phacility/arcanist,folsom-labs/arcanist,sneakpeeq/arcanist,calblueprint/arcanist,fivestars/arcanist,KieranHayes/arcanist,phacility/arcanist,wikimedia/phabricator-arcanist,dguo-coursera/arcanist,phacility/arcanist,Khan/arcanist,dguo-coursera/arcanist,julkiewicz/arcanist,haskell-infra/arcanist,wxstars/arcanist,zhihu/arcanist,calblueprint/arcanist,phacility/arcanist,hach-que/arcanist,alexmv/arcanist,r4nt/arcanist,codelabs-ch/arcanist,dguo-coursera/arcanist,Symplicity/arcanist,memsql/arcanist,eSpark/arcanist,dguo-coursera/arcanist,wharris/arcanist,fivestars/arcanist,wxstars/arcanist,memsql/arcanist,alexmv/arcanist,calblueprint/arcanist,memsql/arcanist,sneakpeeq/arcanist,r4nt/arcanist,uber/arcanist,julkiewicz/arcanist,jra3/arcanist,PageUpPeopleOrg/arcanist-patches,zhihu/arcanist,sneakpeeq/arcanist,fivestars/arcanist,codelabs-ch/arcanist,alexmv/arcanist,coursera/arcanist,jra3/arcanist,fivestars/arcanist,eSpark/arcanist,coursera/arcanist,folsom-labs/arcanist,wikimedia/phabricator-arcanist,alexmv/arcanist,haskell-infra/arcanist,KieranHayes/arcanist,chundongwang/arcanist,KieranHayes/arcanist