Remove traces of xfunc

Hackers,

While playing with the init code I noticed traces of Hellerstein's
"expensive function optimization". It is completely disabled, uses
functions nowhere to be defined, and is out of date. So I removed it.
Here is the patch. Note that it takes out the "pruneable" field from
struct RelOptInfo, since it's not used.

Tom Lane has said a couple of times that he thinks this maybe can be
resurrected; but even if it is, most likely it won't use this code
(what code? These are only hooks.)

(To the patcher: the file src/backend/lib/lispsort.c can also be removed
after this change)

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"Hoy es el primer día del resto de mi vida"