#ifdef out assorted unused GEQO code.

I'd always assumed that backend/optimizer/geqo/'s remarkably poor
showing on code coverage metrics was because we weren't exercising
it much in the regression tests.  But it turns out that a good chunk
of the problem is that there's a bunch of code that is physically
unreachable (because the calls to it are #ifdef'd out in geqo_main.c)
but is being built anyway.  Making the called code have #if guards
similar to the calling code saves a couple of kilobytes of executable
size and should make the coverage numbers more reflective of reality.

It's arguable that we should just delete all the unused recombination
mechanisms altogether, but I didn't feel a need to go that far today.

src/include/optimizer/geqo.h

@@ -31,7 +31,7 @@
  #define GEQO_DEBUG
  */
 
-/* recombination mechanism */
+/* choose one recombination mechanism here */
 /*
  #define ERX
  #define PMX