Remove references to run times for benchmarks.

The numbers are 20 years out of date. For example, a comment stated that
the grand.test target used to take approxiamtely 4 hours to run. On my
5-year-old laptop, the same tests no take ~2 minutes for the bigloo
pre-compiled interpreter, and ~5 minutes for mit and gsi.

The old numbers are still available in Makefile.orig for the curious.
Unify the grand.test and tmp.grand.test targets.