Removes usage of TARGET_NO_VENDOR_BOOT.

- true -> PRODUCT_BUILD_VENDOR_BOOT_IMAGE=false
- false -> (removed, uses default option which builds vendor-boot.img
            if BOARD_BOOT_HEADER_VERSION>=3)

Bug: 170423509
Test: Inspect build artifacts from treehugger.
Change-Id: Id16fbda7fdfc70d62db389d6efb5201cb0d6d4e3