Remove debug_print_rel and replace usages with pprint

Remove debug_print_rel and replace usages with pprint

Going by c4a1933b4, b33ef397a and 05893712c (to name just a few), it seems
that maintaining debug_print_rel() is often forgotten.  In the case of
c4a1933b4, it was several years before anyone noticed that a path type
was not handled by debug_print_rel().  (debug_print_rel() is only
compiled when building with OPTIMIZER_DEBUG).

After a quick survey on the pgsql-hackers mailing list, nobody came
forward to admit that they use OPTIMIZER_DEBUG.  So to prevent any future
maintenance neglect, let's just remove debug_print_rel() and have
OPTIMIZER_DEBUG make use of pprint() instead (as suggested by Tom Lane).
If anyone wants to come forward to claim they make use of
OPTIMIZER_DEBUG in a way that they need debug_print_rel() then they have
around 10 months remaining in the v17 cycle where we could revert this.
If nobody comes forward in that time, then we can likely safely declare
debug_print_rel() as not worth keeping.

Discussion: https://postgr.es/m/CAApHDvoCdjo8Cu2zEZF4-AxWG-90S+pYXAnoDDa9J3xH-OrczQ@mail.gmail.com