scripts: Remove -appveyor version prefix from NuGet packages

This was originally intended as a safety measure to avoid mixing
packages built via AppVeyor vs packages built via other means, but this
just creates friction because of the need to edit the version field
before publishing the package.

Author: zeux

scripts/nuget/pugixml.nuspec

@@ -2,7 +2,7 @@
 <package xmlns="http://schemas.microsoft.com/packaging/2011/10/nuspec.xsd">
   <metadata>
     <id>pugixml</id>
-    <version>1.15.0-appveyor</version>
+    <version>1.15.0</version>
     <title>pugixml</title>
     <authors>Arseny Kapoulkine</authors>
     <owners>Arseny Kapoulkine</owners>