etcdctl: remove GetUser check before mutable commands
etcdctl: remove GetUser check before mutable commands

etcdctl was checking if the user exists before applying mutable calls;
if etcdctl contacts a minority member, the member may not know the user
exists on the cluster yet, causing command failure when it should succeed.

If the user does not exist, it will be picked up once the command goes
through raft.

Fixes #6932

mqliang/etcd,ajityagaty/etcd,coreos/etcd,fanminshi/etcd,qqsun8819/etcd,lorneli/etcd,li-ang/etcd,handsomesun/etcd,insequent/etcd,rnaveiras/etcd,gyuho/etcd,etcd-io/etcd,rnaveiras/etcd,hongchaodeng/etcd,etcd-io/etcd,whuwxl/etcd,vimalk78/etcd,yudai/etcd,heyitsanthony/etcd,whuwxl/etcd,coreos/etcd,rchicoli/etcd,cockroachdb/etcd,mkumatag/etcd,lorneli/etcd,li-ang/etcd,insequent/etcd,sebrandon1/etcd,lumjjb/etcd,akauppi/etcd,fanminshi/etcd,bluebreezecf/etcd,rnaveiras/etcd,joshix/etcd,bdarnell/etcd,dannysauer/etcd,xiang90/etcd,cockroachdb/etcd,visheshnp/etcd,armstrongli/etcd,ajityagaty/etcd,plasticbox/etcd,sinsharat/etcd,mqliang/etcd,tguangch/etcd,mickep76/etcd,qqsun8819/etcd,dannysauer/etcd,gyuho/etcd,bdarnell/etcd,CrazyJvm/etcd,ajityagaty/etcd,bluebreezecf/etcd,a-robinson/etcd,a-robinson/etcd,CrazyJvm/etcd,yudai/etcd,YuleiXiao/etcd,rchicoli/etcd,joshix/etcd,mqliang/etcd,etcd-io/etcd,visheshnp/etcd,lumjjb/etcd,glevand/coreos--etcd,sinsharat/etcd,vimalk78/etcd,akauppi/etcd,mickep76/etcd,mkumatag/etcd,ravigadde/etcd,mkumatag/etcd,sebrandon1/etcd,heyitsanthony/etcd,YuleiXiao/etcd,plasticbox/etcd,tguangch/etcd,jlamillan/etcd,hongchaodeng/etcd,jlamillan/etcd,glevand/coreos--etcd,ravigadde/etcd,xiang90/etcd,armstrongli/etcd,handsomesun/etcd