chore: remove expired nuget sources

Author: jason-aelf

nuget.config

@@ -4,6 +4,5 @@
         <add key="nuget.org" value="https://api.nuget.org/v3/index.json" protocolVersion="3"/>
         <add key="aelf" value="https://www.myget.org/F/aelf-project-dev/api/v3/index.json" protocolVersion="3"/>
         <add key="Nuget Test" value="https://int.nugettest.org/api/v2" />
-        <add key="nexus" value="https://nexus-poc.test.aelf.dev/repository/nuget-group/index.json" protocolVersion="3" />
     </packageSources>
 </configuration>