Fix confusion in comments about generate_gather_paths

Fix confusion in comments about generate_gather_paths

d2d8a229bc58 introduced a new function generate_useful_gather_paths to
be used as a replacement for generate_gather_paths, but forgot to update
a couple of places that referenced the older function.

This is possibly not 100% complete (ref. create_ordered_paths), but it's
better than not changing anything.

Author: "Hou, Zhijie" <houzj.fnst@cn.fujitsu.com>
Reviewed-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Discussion: http://postgr.es/m/4ce1d5116fe746a699a6d29858c6a39a@G08CNEXMBPEKD05.g08.fujitsu.local