Simplify create_merge_append_path for clarity

Simplify create_merge_append_path for clarity

We don't currently support parameterized MergeAppend paths: there's
little use for an ordered path on the inside of a nestloop.  Given
this, we can simplify create_merge_append_path by directly setting
param_info to NULL instead of calling get_appendrel_parampathinfo.  We
can also simplify the Assert for child paths a little bit.

This change won't make any measurable difference in performance; it's
just for clarity's sake.

Author: Richard Guo
Reviewed-by: Alena Rybakina, Paul A Jungwirth
Discussion: https://postgr.es/m/CAMbWs4_n1bgH2nACMuGsXZct3KH6PBFS0tPdQsXdstRfyxTunQ@mail.gmail.com