arm: Delete the unused onKvmExitHypercall method.

arm: Delete the unused onKvmExitHypercall method.

The KVM_EXIT_HYPERCALL KVM exit is now unused, and so even if this
exit handler was plumbed to receive these exits, they would probably
never come.

Change-Id: Ic3ecc789102e761a6dbe80caaf57d61dd95f70a6
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23746
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>