x86: Stop manually clearing RFLAGS.RF after a system call.

x86: Stop manually clearing RFLAGS.RF after a system call.

The system call stub KVM uses in SE mode to call the system call
pseudo instruction which ultimately calls m5Syscall already uses
sysret, and the implementation of sysret clears both the RF and VM bits
itself. There's no reason to do that again explicitly here.

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

Change-Id: Id7b5417564e3f3492ba6efb8ed36fab2f4c38e09
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23175
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>