mips,riscv: Get rid of some Alpha cruft in these System classes.

mips,riscv: Get rid of some Alpha cruft in these System classes.

The consolePanicEvent pointer and addConsoleFuncEvent template were
inherited from Alpha and were not used (and probably make no sense) for
MIPS or RISCV which (to my knowledge) don't have the idea of a
"console" binary.

Change-Id: I109b866a65f69c7334062f7304c7b18acc51d99d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21782
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>