Delete obsolete GC debugging code

Delete obsolete GC debugging code

This code was introduced in 2014 to catch a GC bug that, according to
Paul Eggert in 2019, "seems to have been fixed" (see 2b552f34892
2019-08-21 "Don’t debug fset by default").  It has been marked
obsolete since that time, and no one has mentioned it on our mailing
lists since.  Let's just get rid of it.

* src/alloc.c
(SUSPICIOUS_OBJECT_CHECKING) [ENABLE_CHECKING]: Don't define.
(suspicious_free_record, suspicious_objects, suspicious_object_index)
(suspicious_free_history, suspicious_free_history_index)
(note_suspicious_free) [SUSPICIOUS_OBJECT_CHECKING]: Delete.
(find_suspicious_object_in_range)
(detect_suspicious_free): Delete functions.
(cleanup_vector)
(allocate_vectorlike): Don't call above deleted functions.
(Fsuspicious_object): Delete DEFUN.
(syms_of_alloc) <Ssuspicious_object>: Delete defsubr.