I've certainly used "_oneway_=yes" on inland waterways to document signed traffic flow control, so a blanket removal would make no sense.
There may be places where a previous mapper has tried to use it in error to indicate water flow direction, but you'd need to ask whoever the previous mapper was in each case (or use a bit of common sense).
According to the waterway=stream wiki page (http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dstream):
As of today there is a very small percentage of streams (17593 ways according to taginfo, 0.23%) with _oneway_=yes.
Is there any undocumented purpose? Is it ok and safe to delete
_oneway_=yes tags for streams?
The same question can apply to drains, ditches, canals...
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