Remove useless lookup of root partitioned rel in ExecInitModifyTable().

node->partitioned_rels is only set in UPDATE/DELETE cases, but
ExecInitModifyTable only uses its "rel" variable in INSERT cases,
so the extra logic to find the root rel is just a waste of complexity
and cycles.

Etsuro Fujita, reviewed by Amit Langote

Discussion: https://postgr.es/m/93cf9816-2f7d-0f67-8ed2-4a4e497a6ab8@lab.ntt.co.jp