streams: fix regression in `unpipe()`

Since 2e568d95 there is a bug where unpiping a stream
from a readable stream that has `_readableState.pipesCount > 1`
will cause it to remove the first stream in the
`_.readableState.pipes` array no matter where in the list the
`dest` stream was.

This patch corrects that problem.

PR-URL: https://github.com/nodejs/node/pull/9171
Fixes: https://github.com/nodejs/node/issues/9170


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>