Remove explicit env var definition

We don't need it any more since we removed it from the web UI settings of appveyor.

Author: AArnott

appveyor.yml

@@ -21,10 +21,6 @@ before_build:
     #$blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))
 build_script:
 - ps: >-
-    Write-Host "VisualStudioVersion = $env:VisualStudioVersion"
-
-    $env:VisualStudioVersion = '15.0'
-
     .\build.ps1
 artifacts:
 - path: bin\**\*.nupkg