Simplify API for initially hooking custom-path providers into the planner.

Instead of register_custom_path_provider and a CreateCustomScanPath
callback, let's just provide a standard function hook in set_rel_pathlist.
This is more flexible than what was previously committed, is more like the
usual conventions for planner hooks, and requires less support code in the
core.  We had discussed this design (including centralizing the
set_cheapest() calls) back in March or so, so I'm not sure why it wasn't
done like this already.

src/include/optimizer/pathnode.h

@@ -128,15 +128,6 @@ extern Path *reparameterize_path(PlannerInfo *root, Path *path,
 					Relids required_outer,
 					double loop_count);
 
-/*
- * Interface definition of custom-scan providers
- */
-extern void register_custom_path_provider(const CustomPathMethods *cpp_methods);
-
-extern void create_customscan_paths(PlannerInfo *root,
-						RelOptInfo *baserel,
-						RangeTblEntry *rte);
-
 /*
  * prototypes for relnode.c
  */