Remove variableDefinedExpr

VariableDefinedExpr was under-developed, and would not
take into account if a variable was from the globals
or product config dictionary.

It also always emitted `g.get("VARIABLE") != None`, which
is not correct behavior. In this example makefile:

```
MY_VAR :=

ifdef MY_VAR
$(info MY_VAR is defined)
else
$(info MY_VAR is not defined)
endif

ifdef MY_UNDEFINED_VAR
$(info MY_UNDEFINED_VAR is defined)
else
$(info MY_UNDEFINED_VAR is not defined)
endif

MY_VAR ?= true
MY_UNDEFINED_VAR ?= true


$(info MY_VAR after ?= is $(MY_VAR))
$(info MY_UNDEFINED_VAR after ?= is $(MY_UNDEFINED_VAR))


.PHONY: all
all:
	@:
```

We get the output:

MY_VAR is not defined
MY_UNDEFINED_VAR is not defined
MY_VAR after ?= is
MY_UNDEFINED_VAR after ?= is true

So we can see that even if a variable was set, it's considered
not defined if it was set to an empty value. However, ?= works
differently, and does require the != None, so that was left
as is.

Just use a variableRefExpr and rely on the fact
that variables will be truthy if they're defined.

Fixes: 216700361
Test: go test
Change-Id: If8944da2579e8658e3fc4f666b1f3b2815f8c8b1

mk2rbc/expr.go

@@ -378,32 +378,6 @@ func (eq *eqExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) st
 	}
 }
 
-// variableDefinedExpr corresponds to Make's ifdef VAR
-type variableDefinedExpr struct {
-	v variable
-}
-
-func (v *variableDefinedExpr) emit(gctx *generationContext) {
-	if v.v != nil {
-		v.v.emitDefined(gctx)
-		return
-	}
-	gctx.writef("%s(%q)", cfnWarning, "TODO(VAR)")
-}
-
-func (_ *variableDefinedExpr) typ() starlarkType {
-	return starlarkTypeBool
-}
-
-func (v *variableDefinedExpr) emitListVarCopy(gctx *generationContext) {
-	v.emit(gctx)
-}
-
-func (v *variableDefinedExpr) transform(transformer func(expr starlarkExpr) starlarkExpr) starlarkExpr {
-	// TODO: VariableDefinedExpr isn't really an expression?
-	return v
-}
-
 type listExpr struct {
 	items []starlarkExpr
 }