Revert "Fixed NuGet.config for access to prereleases."

This reverts commit 549d9b5.

Author: AndyButland

NuGet.config

@@ -1,8 +1,22 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
     <add key="Umbraco Prereleases" value="https://www.myget.org/F/umbracoprereleases/api/v3/index.json" />
     <add key="Umbraco Nightly" value="https://www.myget.org/F/umbraconightly/api/v3/index.json" />
   </packageSources>
+  <packageSourceMapping>
+    <packageSource key="nuget.org">
+      <package pattern="*" />
+    </packageSource>
+    <packageSource key="Umbraco Prereleases">
+      <package pattern="Umbraco.Cms.*" />
+      <package pattern="Umbraco.Deploy.*" />
+    </packageSource>
+    <packageSource key="Umbraco Nightly">
+      <package pattern="Umbraco.Cms.*" />
+      <package pattern="Umbraco.Deploy.*" />
+    </packageSource>
+  </packageSourceMapping>
 </configuration>