Retire has_multiple_baserels()

The function has_multiple_baserels() is used in set_subquery_pathlist()
to check and see if there are more than 1 base rel, by looping through
simple_rel_array[]. I think one simpler way to do that is to leverage
root->all_baserels by

bms_membership(root->all_baserels) == BMS_MULTIPLE

all_baserels is computed in deconstruct_jointree (v16) or in
make_one_rel (v15 and earlier), both are before we generate access paths
for subquery RTEs, and it contains all base rels (but not "other" rels).
So it should be a suitable replacement. I doubt that there would be any
measurable performance gains. So please consider it cosmetic.

I've attached a patch to do that. Any thoughts?

Thanks
Richard