If you are running SSMS 2017 at least v17.6, there is now an option to turn this off. In the options dialog, go to "SQL Server Object Explorer" and then to "Commands". Under the "Drag/Drop" heading is a new option "Surround object names with brackets when dragged". By default, this is set to True, so just change it to False. SSMS may need to be restarted to make the change effective.
Note: Any objects that need the brackets (eg. with spaces or other whitespace/non-alphanumeric characters, reserved keywords) will still have brackets when drag/dropped.