orswot doesn't remove deleted entries on merge

The `riak_dt_orswot` CRDT keeps deleted entries after they've been removed if the clocks haven't been incremented by an add operation. Example test case:

```
    {ok, A1} = riak_dt_orswot:update({add, <<"foo">>}, 1, riak_dt_orswot:new()),
    {ok, A2} = riak_dt_orswot:update({add, <<"bar">>}, 1, A1),
    {ok, B1} = riak_dt_orswot:update({add, <<"baz">>}, 2, riak_dt_orswot:new()),
    C = riak_dt_orswot:merge(A2, B1),
    {ok, A3} = riak_dt_orswot:update({remove, <<"bar">>}, 1, A2),
    D = riak_dt_orswot:merge(A3, C),
    % Actual return value is [<<"bar">>,<<"baz">>,<<"quux">>]
    [<<"baz">>,<<"foo">>] == riak_dt_orswot:value(D).
```