build_mixed: removed product.img if system_dir does not have one.

When '-s' option is used but the system_dir does not have a
product.img, removed existing product.img from the device image.

Bug: 120578242
Test: build_mixed -s system_dir device_dir out_dir checkvintf
Change-Id: Ib669763847f3661596a74a6833b2030685d58ad5