sim: Get rid of the now unused getSyscallArg method.

sim: Get rid of the now unused getSyscallArg method.

Change-Id: I2f78420d8687da7530feb66784fe3e6d2357baf8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23462
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>