Eliminate 'comparison is always false' static analyzer warning in finalize

Eliminate 'comparison is always false' static analyzer warning in finalize

GC_enqueue_all_finalizers code refactoring is done (removal of the
unreachable statement) to eliminate the warning.

* finalize.c (GC_enqueue_all_finalizers): Remove "prev_fo" local
variable; remove "register" keyword for local variables;
remove the pointer to the chain of hash table entries from the roots
(i.e. setting the roots pointer to null) at the beginning of processing
the chain (instead of updating the roots pointer on deletion of each
entry and finally setting it to null); remove fo_set_next(prev_fo) as
it is never called (because prev_fo was always null); update
GC_fo_entries only when the whole table processed (i.e. all items
removed).