Benchmark.t: remove CPU-speed-sensitive test

Benchmark.t has been randomly failing test 15 in smokes for ages.
This is the one that checks that a loop run 3*N times burns approximately
3 times more CPU than when run just N times.

For the last month the test has included a calibration loop and test,
which does much the same thing, but without using any code from
Benchmark.pm. This has failed just as much, which confirms that its an
issue with the smoke host (such as a variable speed CPU or whatever),
rather than any flaw in the Benchmark.pm library logic.

So just remove the calibration loop and the dodgy test.