checker_event has a clone vfunc implemented by all the concrete
subclasses, but this is never used (a holdover from a very early
implementation).  This patch deletes it.

gcc/analyzer/ChangeLog:
	* checker-path.h (checker_event::clone): Delete vfunc decl.
	(debug_event::clone): Delete vfunc impl.
	(custom_event::clone): Delete vfunc impl.
	(statement_event::clone): Delete vfunc impl.
	(function_entry_event::clone): Delete vfunc impl.
	(state_change_event::clone): Delete vfunc impl.
	(start_cfg_edge_event::clone): Delete vfunc impl.
	(end_cfg_edge_event::clone): Delete vfunc impl.
	(call_event::clone): Delete vfunc impl.
	(return_event::clone): Delete vfunc impl.
	(setjmp_event::clone): Delete vfunc impl.
	(rewind_from_longjmp_event::clone): Delete vfunc impl.
	(rewind_to_setjmp_event::clone): Delete vfunc impl.
	(warning_event::clone): Delete vfunc impl.