build_image: Remove mkerofsimage.sh.

These wrapper scripts are difficult to maintain and there's not really
any value add. The argument strings don't match and every new build flag
has to be connected in an extra place.

Bug: 222715577
Test: make with EROFS enabled
Change-Id: Ie0e51cc30aa08b004d5d8345a2a2d885e193ffa9