FAB-853

Ledger is removed from
   . core/peer/peer.go
   . core/rest/api.go

Ledger package itself is removed. "ledgernext" is
replaced with "ledger".

Change-Id: Ie7dfbd9bf94afa0031aef759fc46929e4fb3b400
Signed-off-by: Srinivasan Muralidharan <muralisr@us.ibm.com>