Clean up RandomBitsSupplier.

The reason the array had later items commented out is that the loop
was wrong: it would always choose the _last_ valid device, not the
_first_ as intended.

There were two choices here:
1. fix the loop and have the same list of three devices that OpenSSL uses.
2. remove the loop and just say "you must have /dev/urandom".

This patch goes with option 2.

Change-Id: Ia99be6ba929c7b6c35bed77c19d47de80719973c