util/hash_table: do not leak u64 struct key
For non 64bit devices the key stored in hash_table_u64 is wrapped in hash_key_u64 structure, which is never free.
This commit fixes this issue by just removing the user-defined
delete_function
parameter in hash_table_u64_{destroy,clear} (which
nobody is using) and using instead a delete function to free this
structure.