Revert "Fix GetEntryAssemblyLocation Uri (#4799)"

This reverts commit 9cdfdc3.

Author: ryalanms

src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/AppModel/ContentFilePart.cs

@@ -73,7 +73,8 @@ protected override Stream GetStreamCore(FileMode mode, FileAccess access)
                 BaseUriHelper.GetAssemblyNameAndPart(Uri, out filePath, out assemblyName, out assemblyVersion, out assemblyKey);
 
                 // filePath should not have leading slash.  GetAssemblyNameAndPart( ) can guarantee it.
-                _fullPath = System.IO.Path.Combine(codeBase.LocalPath, filePath);
+                Uri file = new Uri(codeBase, filePath);
+                _fullPath = file.LocalPath;
             }
 
             stream = CriticalOpenFile(_fullPath);