Remove superfluous CompletionResults.cs file (and fix tooltips)

This removes an abstraction that sat between `SMA.CompletionResults` and
`OmniSharp.CompletionItems`. It most likely existed out of necessity
before a prior refactor that introduced OmniSharp. Now we can skip the
middle layer and instead convert directly, massively simplifying the
code (and fixing tooltips).

Author: andyleejordan