Remove implicit outputs for the flattened APEX Make module

When flattened, the Make module for an APEX is a phony package for the
files in it. There is no output (either implcit or explicit) there.

Bug: 144338929
Test: m
Test: check the generated Android-<target>.mk file
Change-Id: I86df3b69f402a7e2d94bafa39f2aad3312d8c28b