Remove dead code in get_param_path_clause_serials()

Remove dead code in get_param_path_clause_serials()

The function get_param_path_clause_serials() is used to get the set of
pushed-down clauses enforced within a parameterized Path.  Since we
don't currently support parameterized MergeAppend paths, and it
doesn't look like that is going to change anytime soon (as explained
in the comments for generate_orderedappend_paths), we don't need to
consider MergeAppendPath in this function.

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

Author: Richard Guo
Reviewed-by: Andrei Lepikhov
Discussion: https://postgr.es/m/CAMbWs4_Puie4DQ2ODvjQB_3CxYkUODnrJm8jn_ObMAcrjYNW7Q@mail.gmail.com