arch: Get rid of the now unused setSyscallArg.

arch: Get rid of the now unused setSyscallArg.

Setting syscall args isn't really something we need to do in gem5,
since that will be taken care of by the code actually calling the
syscall. We just need to be able to retrieve the value it put there.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: I0bb6d5d0207a7892414a722b3788cb70ee509582
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23174
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>