Remove assertion that we can't register for change notifications twice in SwiftPMWorkspace

There’s no harm in registering for change notifications for the same file twice. Currently, this can happen if you open two files that have the same main file.

 `BuildSystemManager` checks that there are no more files referencing a main file before calling `unregisterForChangeNotifications`, so this is where the “complicated” logic lives that checks if sourcekit-lsp really isn’t interested in a file anymore.

rdar://117603105

Author: ahoppen

Sources/SKSwiftPMWorkspace/SwiftPMWorkspace.swift

@@ -300,7 +300,6 @@ extension SwiftPMWorkspace: SKCore.BuildSystem {
   }
 
   public func registerForChangeNotifications(for uri: DocumentURI) async {
-    assert(!self.watchedFiles.contains(uri), "Registered twice for change notifications of the same URI")
     self.watchedFiles.insert(uri)
   }