Makefile: Remove confusing rule

This rule:
$(BUILD_DIR)/$(GOOS)-$(GOARCH):
	mkdir -p $(BUILD_DIR)/$(GOOS)-$(GOARCH)

may make one think that it will create the right directory in cross
builds, but this is misleading, GOOS/GOARCH are set at the very
beginning of the Makefile, and the name of the directory which is
created is fixed at the time the Makefile is parsed (to linux/amd64/ on
a linux machine for example). As "go build" will create the target
directory if needed, we can get rid of this rule.

Author: cfergeau

Makefile

@@ -79,16 +79,13 @@ binappend:
 $(CURDIR)/bin/crc$(IS_EXE):
 	go install -ldflags="$(VERSION_VARIABLES)" ./cmd/crc
 
-$(BUILD_DIR)/$(GOOS)-$(GOARCH):
-	mkdir -p $(BUILD_DIR)/$(GOOS)-$(GOARCH)
-
-$(BUILD_DIR)/darwin-amd64/crc: $(BUILD_DIR)/$(GOOS)-$(GOARCH)
+$(BUILD_DIR)/darwin-amd64/crc:
 	GOARCH=amd64 GOOS=darwin go build -ldflags="$(LDFLAGS)" -o $(BUILD_DIR)/darwin-amd64/crc ./cmd/crc
 
-$(BUILD_DIR)/linux-amd64/crc:  $(BUILD_DIR)/$(GOOS)-$(GOARCH)
+$(BUILD_DIR)/linux-amd64/crc:
 	GOOS=linux GOARCH=amd64 go build -ldflags="$(LDFLAGS)" -o $(BUILD_DIR)/linux-amd64/crc ./cmd/crc
 
-$(BUILD_DIR)/windows-amd64/crc.exe:  $(BUILD_DIR)/$(GOOS)-$(GOARCH)
+$(BUILD_DIR)/windows-amd64/crc.exe:
 	GOARCH=amd64 GOOS=windows go build -ldflags="$(LDFLAGS)" -o $(BUILD_DIR)/windows-amd64/crc.exe ./cmd/crc
 
 .PHONY: cross ## Cross compiles all binaries