Add clear element to Nuget.config

2. Add `disabledPackageSources` to `nuget.config`
3. Remove useless feed from `nuget.config`

Co-authored-by: wyunchi-ms <yunwang@microsoft.com>

NuGet.Config

@@ -1,9 +1,13 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear/>
+    <!-- Do not add any additional feeds if new packages are needed they need to come from nuget.org or our azure-sdk-for-net DevOps feed -->
     <add key="local-feed" value="tools/LocalFeed" />
     <add key="myget-azure-powershell" value="https://www.myget.org/F/azure-powershell/api/v3/index.json" />
-    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
   </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>

tools/Az.Tools.Predictor/NuGet.Config

@@ -3,6 +3,8 @@
   <packageSources>
     <clear />
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
-    <add key="azure-powershell" value="https://www.myget.org/F/azure-powershell/api/v3/index.json" />
   </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>

tools/NetCoreCsProjSync/NuGet.Config

@@ -1,7 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear/>
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" /> 
-    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
   </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>

tools/NetCorePsd1Sync/NuGet.Config

@@ -1,7 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <configuration>
   <packageSources>
+    <clear/>
     <add key="nuget.org" value="https://api.nuget.org/v3/index.json" /> 
-    <add key="dotnet-core" value="https://dotnet.myget.org/F/dotnet-core/api/v3/index.json" />
   </packageSources>
+  <disabledPackageSources>
+    <clear />
+  </disabledPackageSources>
 </configuration>