only silently drop DOWN-normal messages in deleted modstate

This is a restriction of the modification made in PR #334.

Dropping all `{'DOWN',_,process,_,normal}` messages on the floor instead of passing them to vnode `handle_info` functions causes `riak_pipe` vnodes to missing messages that it uses to cleanup workers for pipes that shutdown unexpectedly.

This commit restricts the DOWN-normal message dropping to the case that the vnode's modstate is `{deleted, _}`. PR #334 suggests the original modification was made only to quiet the log spam generated by the following clause, which also only operates in modstate-deleted.

Before this commit, the `riak_test` `pipe_verify_exceptions` would fail during its `verify_middle_fitting_normal` test, because workers would be left running after the fitting exited `normal`. After this commit, workers are once again terminated correctly, so the test passes again.

This PR resolves basho/riak_test#316.