[#182] Drop GO111MODULE from builds

It is enabled by default since go1.16

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

Makefile

@@ -22,7 +22,6 @@ all: $(BINS)
 $(BINS): $(DIRS) dep
 	@echo "⇒ Build $@"
 	CGO_ENABLED=0 \
-	GO111MODULE=on \
 	go build -v -trimpath \
 	-ldflags "-X main.Version=$(VERSION)" \
 	-o $@ ./
@@ -35,11 +34,9 @@ $(DIRS):
 dep:
 	@printf "⇒ Download requirements: "
 	@CGO_ENABLED=0 \
-	GO111MODULE=on \
 	go mod download && echo OK
 	@printf "⇒ Tidy requirements: "
 	@CGO_ENABLED=0 \
-	GO111MODULE=on \
 	go mod tidy -v && echo OK
 
 docker/%:
@@ -65,7 +62,7 @@ cover:
 # Reformat code
 fmt:
 	@echo "⇒ Processing gofmt check"
-	@GO111MODULE=on gofmt -s -w ./
+	@gofmt -s -w ./
 
 # Build clean Docker image
 image: