bpo-39505: delete the redundant '/' in $env:VIRTUAL_ENV (GH-18290)

Lib/venv/scripts/common/Activate.ps1

@@ -168,7 +168,6 @@ if ($VenvDir) {
 } else {
     Write-Verbose "VenvDir not given as a parameter, using parent directory name as VenvDir."
     $VenvDir = $VenvExecDir.Parent.FullName.TrimEnd("\\/")
-    $VenvDir = $VenvDir.Insert($VenvDir.Length, "/")
     Write-Verbose "VenvDir=$VenvDir"
 }