Clean up EditorFileSystem script parsing

* Optimize only update modified/added/removed files.
* Clean up documentation parsing.

doc/classes/EditorFileSystem.xml

@@ -71,12 +71,6 @@
 				This will not import the file. To reimport, call [method reimport_files] or [method scan] methods.
 			</description>
 		</method>
-		<method name="update_script_classes">
-			<return type="void" />
-			<description>
-				Scans the script files and updates the list of custom class names.
-			</description>
-		</method>
 	</methods>
 	<signals>
 		<signal name="filesystem_changed">