Remove tree-reduce algorithm

Remove tree-reduce algorithm

This was formerly used only on the now-removed MIC platform. It could
well be useful these days with high core counts on conventional CPU
platforms, but it is likely that something else will prove limiting to
MD step thread scaling before that point. If we ever want it back,
then git will remember it, and we'd probably re-implement it on
different infrastructure anyway.

No need for release notes, as it was not something intended for users
to use.

Renamed the "standard" reduction function now that there is no need to
distinguish it from the tree reduction.

Part of #3982
Fixes #3891