Remove unused notice_files target

The notice_files target generates $(TARGET_OUT_INTERMEDIATES)/NOTICE.txt and
$(TARGET_OUT_INTERMEDIATES)/NOTICE.html. The former is not needed and the
latter is an explicit dependency of $(TARGET_OUT_INTERMEDIATES)/NOTICE.html.gz.
We can therefore remove the target and the logic to generate NOTICE.txt.

Change-Id: I6179c049bebe590c7298e8be0a30e30265423468