Don't set nova api/conductor workers since nova handles this

The changes to devstack to set nova api/conductor workers to 4 predated
commit 75c96a48fc7e5dfb59d8258142b01422f81b0253 to default the workers
equal to the number of CPUs, so this change just removes the hard-coding
from devstack and relies on nova's defaults for number of workers.

Change-Id: I384fc9eeb7a7eddf4679a6852203307a92d88a93
Merge "Don't set nova api/conductor workers since nova handles this"