Ignore result of the hashtree destroy call.

Resolves:

riak_kv_index_hashtree.erl:814: Expression produces a value of type 'ok' | hashtree:hashtree(), but this value is unmatched
Fix incorrect specification.

Resolves:

riak_kv_index_hashtree.erl:517: Invalid type specification for function riak_kv_index_hashtree:index_2i_n/0. The success typing is () -> {0,0}