Makefile: the toolchain directive should be better supported now

Even Go 1.21 has improved support for it since 1.21.11
(golang/go#62278)

Signed-off-by: Simon Deziel <simon.deziel@canonical.com>

Makefile

@@ -91,7 +91,6 @@ ifneq "$(LXD_OFFLINE)" ""
 endif
 	go get -t -v -d -u ./...
 	go mod tidy -go=$(GOMIN)
-	go get toolchain@none
 
 	@echo "Dependencies updated"