mem: Get rid of the now unused SecurePortProxy class.

mem: Get rid of the now unused SecurePortProxy class.

This proxy was only used by the ARM semihosting interface which can now
use a tweaked regular TranslatingPortProxy or SETranslatingPortProxy
instead of this special purpose class.

This sort of class would still be necessary if you wanted to use
physical addresses and not virtual addresses, but presently there is no
such use. This code can be retrieved from history if it's needed in the
future.

Change-Id: Ie47a8b4bb173cba1a06bd3ca60391081987936b8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26625
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>