value.c: Remove unused variables

Obvious patch to remove unused local variables (found by adding
-Wunused).  I didn't touch this one in value_fetch_lazy, because
check_typedef could have a desired side-effect.

  3743  struct type *type = check_typedef (value_type (val));

gdb/ChangeLog:

	* value.c (release_value): Remove unused variable.
	(record_latest_value): Likewise.
	(access_value_history): Likewise.
	(preserve_values): Likewise.