When our projects were last updated to a newer version of Microsoft.Net.ToolsetCompilers, NuGet didn't replace the old version--it just put the new version next to it. Every project ended up referencing both versions, and which set of compiler binaries "won" depended on the order they appeared in the project file. So different projects built with different compilers.

This change strips out all references to the old version. (changeset 1352054)