Revert "allow overriding VERSION value in Makefile"

This reverts commit 9d9273c926.

This commit broke the build for several other projects (see comments
here: https://github.com/opencontainers/runc/pull/4270, after the merge)
and we don't really need this to be able to set the version without
changing the file.

With this commit reverted, we can still run:

	make VERSION="1.2.3"

and it just works. It doesn't take it from an env variable, but that is
what broke all the other projects (VERSION is just too generic as an env
var, especially for a project like runc that is embedded in many
others).

Signed-off-by: Rodrigo Campos <rodrigoca@microsoft.com>

Makefile

@@ -18,7 +18,7 @@ BUILDTAGS ?= seccomp urfave_cli_no_docs
 BUILDTAGS += $(EXTRA_BUILDTAGS)
 
 COMMIT ?= $(shell git describe --dirty --long --always)
-VERSION ?= $(shell cat ./VERSION)
+VERSION := $(shell cat ./VERSION)
 LDFLAGS_COMMON := -X main.gitCommit=$(COMMIT) -X main.version=$(VERSION)
 
 GOARCH := $(shell $(GO) env GOARCH)