Revert "Update previews on preference change (#9507)"

The referenced commit lead to performance issues and possibly
crashes in some cases when saving preferences. Thanks to Guillaume
for discovering the issues. See #9828 for more discussion.

I hope to come up with a better fix for #9507.

This reverts commit 390ae05444.

src/BufferList.cpp

@@ -344,15 +344,6 @@ void BufferList::recordCurrentAuthor(Author const & author)
 }
 
 
-void BufferList::updatePreviews()
-{
-	BufferStorage::iterator it = bstore.begin();
-	BufferStorage::iterator end = bstore.end();
-	for (; it != end; ++it)
-		(*it)->updatePreviews();
-}
-
-
 int BufferList::bufferNum(FileName const & fname) const
 {
 	FileNameList const buffers(fileNames());

src/BufferList.h

@@ -115,8 +115,6 @@ public:
 	//@{
 	/// reset current author for all buffers
 	void recordCurrentAuthor(Author const & author);
-	/// update previews for all buffers, e.g. for Prefs update
-	void updatePreviews();
 	/// Call changed() on all buffers, internal or not
 	void changed(bool update_metrics) const;
 	/// emergency save for all buffers

src/frontends/qt4/GuiPrefs.cpp

@@ -3371,8 +3371,6 @@ void GuiPreferences::dispatchParams()
 		update_screen_font_ = false;
 	}
 
-	theBufferList().updatePreviews();
-
 	// The Save button has been pressed
 	if (isClosing())
 		dispatch(FuncRequest(LFUN_PREFERENCES_SAVE));