Delete retain_list_builder.go (#7096)

I was walking the code to try to understand in a bit more detail how the
state root hash is constructed and stumbled across
`retain_list_builder.go` as a consumer of the retain list APIs. But, as
far as I can tell, this file doesn't seem to actually be used anywhere
(including tests), and, it's seen no development (other than import
fixes) since 2020 or so. All linting and tests still pass for me locally
without it, so, I believe it's safe to simply remove.

Co-authored-by: Jason Yellick <[email protected]>