Remove code to run GC more frequently as process image size increased

Remove code to run GC more frequently as process image size increased

* libguile/gc.c: Remove code that would try to run GC more frequently as
  the process image size was increasing.  Before, it was often the case
  that the heap was the main component of image size, but with
  expandable stacks and statically allocated data that is no longer
  true.  Also, once scm_gc_register_allocation was incorporated, we
  don't need to be so conservative any more.  It seems this code was
  simply causing too many gc's to run.  Removing it improves some
  micro-benchmarks; time will tell.