Remove Nuget binary from sources

[karolz-ms]

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.

[xiaomi7732]

Lesson learned. @karolz-ms, have you tried to use this build step to bring in the binary:

        - task: NuGetToolInstaller@1
          inputs:
            versionSpec: 
            checkLatest: true