pgbench: Remove \setrandom.

pgbench: Remove \setrandom.

You can now do the same thing via \set using the appropriate function,
either random(), random_gaussian(), or random_exponential(), depending
on the desired distribution.  This is not backward-compatible, but per
discussion, it's worth it to avoid having the old syntax hang around
forever.

Fabien Coelho, reviewed by Michael Paquier, and adjusted by me.