nir: Remove shared support from lower_io

nir: Remove shared support from lower_io

No drivers are using this anymore so we can delete it and not keep
maintaining this legacy code-path.  If any drivers want this in the
future, they should use nir_lower_varst_to_explicit_types followed by
nir_lower_explicit_io.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5418>