Revert "Clangd: Avoid client restart after modifying open documents"

The whole point of tracking external changes to files is to use the open
files as a sentinel to indicate that non-open files (on which we don't
have a watch) have likely also been changed.
This reverts commit cff26d813a.

Change-Id: I5a8b3e6709eda881b912916cf0838b45d1ff4fa4
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/plugins/clangcodemodel/clangmodelmanagersupport.cpp

@@ -796,11 +796,6 @@ void ClangModelManagerSupport::watchForExternalChanges()
         if (!LanguageClientManager::hasClients<ClangdClient>())
             return;
         for (const FilePath &file : files) {
-            if (TextEditor::TextDocument::textDocumentForFilePath(file)) {
-                // if we have a document for that file we should receive the content
-                // change via the document signals
-                continue;
-            }
             const ProjectFile::Kind kind = ProjectFile::classify(file.toString());
             if (!ProjectFile::isSource(kind) && !ProjectFile::isHeader(kind))
                 continue;