kern/subr_unit: Rip srandomdev, random(3) out of dead code

The simulation cannot be reproduced, so the value of using a deterministic PRNG
like random(3) is dubious. The number of repitions used in the sample isn't a
problem for the Chacha implementation of arc4random we have today. (Also, no
one actually runs this code; it was provided as an example of the work the
author did validating the implementation. It's not even test code.)