Remove Nuget binary from sources

We have a Nuget binary checked in into our sources (`tools` folder). A better way would be to rely on a copy that exists in the build environment.

The binary is used by `tools\restore.cmd` script. A straightforward attempt to replace the use with `dotnet restore` failed--the latter does not seem to properly restore the old-style signing projects that we use for binary and Nuget package signing.