Remove typo'd variables

LOCAL_DEX_PREOPT_IMAGE_LOCATION := $(TARGET_CORE_IMG_OUT) doesn't
do anything because TARGET_CORE_IMG_OUT is never set.

LOCAL_DEX_PREOPT_IMAGE := $(HOST_CORE_IMG_LOCATION) doesn't do
anything because nothing reads LOCAL_DEX_PREOPT_IMAGE.

Neither seems useful with LOCAL_DEX_PREOPT := false.

Test: m checkbuild
Change-Id: I26c0a848e15784c4dad6f8c3bb6786b98ac85c20