cpu: Remove the "SingleThreaded" fetch policy from the O3 CPU.

cpu: Remove the "SingleThreaded" fetch policy from the O3 CPU.

The fetch policy is only meaningful for SMT simulations. The
"SingleThreaded" value is a placeholder which is the default, and is
only supposed to be used in non-SMT simulations.

Rather than have this enum value and have special checks for it in
various places in O3, we can just eliminate it and set the default,
which is still only meaningful in SMT simulations, be an SMT fetch
policy.

The DerivO3CPUParams::create() function would forcefully change the
the fetch policy from "SingleThreaded" to "RoundRobin" anyway if there
were more than one thread, so that can be the actual default instead of
the shadow effective default.

Change-Id: I458fda00b5bcc246b0957e6c937eab0c5b4563c3
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35935
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>