base: Get rid the Callback type.

base: Get rid the Callback type.

This leaves only the lambda/std::function based CallbackQueue2, soon to
be renamed just CallbackQueue.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: I4e2fd3b7b684c414be6db0e268284ab63e6cfdff
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32650
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>