multi: Remove GO111MODULE env variable in Makefile

The `GO111MODULE` variable is not required from go 1.16
https://go.dev/blog/go116-module-changes

lnrpc/Dockerfile

@@ -19,7 +19,6 @@ ENV GOMODCACHE=/tmp/build/.modcache
 RUN cd /tmp \
   && mkdir -p /tmp/build/.cache \
   && mkdir -p /tmp/build/.modcache \
-  && export GO111MODULE=on \
   && go install google.golang.org/protobuf/cmd/protoc-gen-go@${PROTOBUF_VERSION} \
   && go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@${PROTOC_GEN_GO_GRPC_VERSION} \
   && go install github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-grpc-gateway@${GRPC_GATEWAY_VERSION} \