Refactor idemix implementation (#2955)

This PR removes the internal implementation of the idemix protocol and uses
an external dependency that contains the same implementation.

Signed-off-by: Alessandro Sorniotti <aso@zurich.ibm.com>

Makefile

@@ -25,7 +25,6 @@
 #   - docs - builds the documentation in html format
 #   - gotools - installs go tools like golint
 #   - help-docs - generate the command reference docs
-#   - idemixgen - builds a native idemixgen binary
 #   - integration-test-prereqs - setup prerequisites for integration tests
 #   - integration-test - runs the integration tests
 #   - ledgerutil - builds a native ledgerutil binary
@@ -85,14 +84,13 @@ GO_TAGS ?=
 RELEASE_EXES = orderer $(TOOLS_EXES)
 RELEASE_IMAGES = baseos ccenv orderer peer tools
 RELEASE_PLATFORMS = darwin-amd64 linux-amd64 windows-amd64
-TOOLS_EXES = configtxgen configtxlator cryptogen discover idemixgen ledgerutil osnadmin peer
+TOOLS_EXES = configtxgen configtxlator cryptogen discover ledgerutil osnadmin peer
 
 pkgmap.configtxgen    := $(PKGNAME)/cmd/configtxgen
 pkgmap.configtxlator  := $(PKGNAME)/cmd/configtxlator
 pkgmap.cryptogen      := $(PKGNAME)/cmd/cryptogen
 pkgmap.discover       := $(PKGNAME)/cmd/discover
-pkgmap.idemixgen      := $(PKGNAME)/cmd/idemixgen
-pkgmap.ledgerutil		  := $(PKGNAME)/cmd/ledgerutil
+pkgmap.ledgerutil     := $(PKGNAME)/cmd/ledgerutil
 pkgmap.orderer        := $(PKGNAME)/cmd/orderer
 pkgmap.osnadmin       := $(PKGNAME)/cmd/osnadmin
 pkgmap.peer           := $(PKGNAME)/cmd/peer