mem: Remove the now unused Copy* methods from the FS port proxy.

Change-Id: Ie433a9e4c9ee748911060eb7b1b47e617aa297a6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18576
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>