Remove _API_FILE variables

There are no known usages of these variables, and they've been
incorrectly set for a long time.

The phony for $(LOCAL_MODULE) in droiddoc to build the api txt
has been moved to soong.

Bug: 152479829
Test: m
Change-Id: I31742e50d1af42fb899c7a3436af5647bd90cc47
Merged-In: I31742e50d1af42fb899c7a3436af5647bd90cc47