roman-numerals: rm untested example convertAlt
roman-numerals: rm untested example convertAlt

Before https://github.com/exercism/d/pull/100, the example tested both
`convert` and `convertAlt`.
After that PR, instead the example only tests `convert`, leaving
`convertAlt` untested.

Leaving untested code in this repo runs the risk of the code going out
of date.

Delete the untested implementation. The goal of the track repo is not to
showcase a variety of working solutions; the track only need demonstrate
a single working solution.

Between `convert` and `convertAlt`, `convert` was arbitrarily chosen to
remain, so `convertAlt` is to be deleted.

exercism/xdlang