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