doc: remove duplicate aws-ec2 menu item
doc: remove duplicate aws-ec2 menu item

the auth backends menu had a duplicate entry for aws-ec2 auth.
removed the dup one.
Caiyeon/vault,lirao/vault,chrishoffman/vault,mymercurialsky/vault,peteryhwong/vault,memory/vault,jgcallero/vault,peteryhwong/vault,cimarron-pistoncloud/vault,joelthompson/vault,jam-pot/vault,tallpauley/vault,danielsomerfield/vault,doodles526/vault,joelthompson/vault,Banno/vault,jam-pot/vault,jgcallero/vault,hashicorp/vault,chrishoffman/vault,MonsantoCo/vault,rnaveiras/vault,lemondrank/vault,Aloomaio/vault,Caiyeon/vault,hashicorp/vault,rnaveiras/vault,quixoten/vault,kingland/vault,rnaveiras/vault,joelthompson/vault,deverton/hashicorp-vault,cimarron-pistoncloud/vault,ryon/vault,wjam/vault,memory/vault,rnaveiras/vault,michaelansel/vault,jmhardison/vault,LolalyLuo/vault,hartsock/vault,michaelansel/vault,mgk/vault,hashicorp/vault,lirao/vault,hartsock/vault,rnaveiras/vault,jmhardison/vault,doodles526/vault,LolalyLuo/vault,naunga/vault,hartsock/vault,joelthompson/vault,naunga/vault,Aloomaio/vault,quixoten/vault,mgk/vault,kingland/vault,jmhardison/vault,jmhardison/vault,Aloomaio/vault,lemondrank/vault,hartsock/vault,Banno/vault,hartsock/vault,tallpauley/vault,mgk/vault,hashicorp/vault,Aloomaio/vault,ryon/vault,danielsomerfield/vault,mymercurialsky/vault,martin-ducar-gd/vault,quixoten/vault,martin-ducar-gd/vault,kingland/vault,hashicorp/vault,deverton/hashicorp-vault,hashicorp/vault,mgk/vault,wjam/vault,kingland/vault,quixoten/vault,jmhardison/vault,kingland/vault,MonsantoCo/vault