How to fix losing System.Data.dll in Project?

Thank you @incorrect, your second comment actually brought me on the right track. The last post in that thread refers to this page, where this problem is solved (but if I interpret it correctly, they will not fix this as it is not a bug). If you want, you can just repost this as an answer and I will delete mine and accept yours :)

Basically, you need to add the following code in your Assets/Plugins/Editor folder. You can also modify it to work with other DLLs.

 using System;
 using System.IO;
 using System.Linq;
 using System.Text;
 using System.Xml.Linq;
 using SyntaxTree.VisualStudio.Unity.Bridge;
 using UnityEditor;
 
 namespace Assets.Plugins.Editor
 {
     [InitializeOnLoad]
     public class ProjectFileHook
     {
         // necessary for XLinq to save the xml project file in utf8
         class Utf8StringWriter : StringWriter
         {
             public override Encoding Encoding
             {
                 get { return Encoding.UTF8; }
             }
         }
 
         static void ProcessNodesWithIncludeAttribute(XDocument document, string localName, string includeValue, Action<XElement> action)
         {
             var nodes = document
                 .Descendants()
                 .Where(p => p.Name.LocalName == localName);
 
             foreach (var node in nodes)
             {
                 var xa = node.Attribute("Include");
                 if (xa != null && !string.IsNullOrEmpty(xa.Value) && string.Equals(xa.Value, includeValue))
                 {
                     action(node);
                 }
             }
         }
 
         // Remove System.Data from project (not from file system so Unity can compile properly)
         static void RemoveFileFromProject(XDocument document, string fileName)
         {
             ProcessNodesWithIncludeAttribute(document, "None", fileName, element => element.Remove());
         }
 
         // Adjust references, by using the default framework assembly instead of local file (remove the HintPath)
         static void RemoveHintPathFromReference(XDocument document, string assemblyName)
         {
             ProcessNodesWithIncludeAttribute(document, "Reference", assemblyName, element => element.Nodes().Remove());
         }
 
         static ProjectFileHook()
         {
             ProjectFilesGenerator.ProjectFileGeneration += (string name, string content) =>
             {
                 var document = XDocument.Parse(content);
 
                 RemoveFileFromProject(document, @"Assets\System.Data.dll");
                 RemoveHintPathFromReference(document, "System.Data");
 
                 var str = new Utf8StringWriter();
                 document.Save(str);
 
                 return str.ToString();
             };
         }
     }
 }