Revert "nir: Add a couple trivial abs optimizations"

Revert "nir: Add a couple trivial abs optimizations"

These were already added in 9fdaeb7776c ("nir: add min/max optimisation"),
and there's no point in doing them twice.

This reverts commit e4d346c86db0ae332fcdf55eac0e075cfb99a7eb.

Fixes: e4d346c86db ("nir: Add a couple trivial abs optimizations")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3786>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3786>