[FAB-14369]Fix dev mode failing to build Chaincode

Building Chaincode fails with the ccenv container created from
Alpine because go build process can't import the shim package.
GOPATH in ccenv changed from "/opt/gopath" to "/go" due to the
move to Alpine. However, docker-compose-simple.yaml still has
`GOPATH=/opt/gopath`.
This CR removes the environment variable.

FAB-14369 #done

Change-Id: I18d9861d9b6f45c1548169fbb143e7883f8ba207
Signed-off-by: Yuki Kondo <yuki.kondo@hal.hitachi.com>

chaincode-docker-devmode/docker-compose-simple.yaml

@@ -70,7 +70,6 @@ services:
     image: hyperledger/fabric-ccenv
     tty: true
     environment:
-      - GOPATH=/opt/gopath
       - CORE_VM_ENDPOINT=unix:///host/var/run/docker.sock
       - FABRIC_LOGGING_SPEC=DEBUG
       - CORE_PEER_ID=abstore