linux(4): Drop a weird comment from linux_set_syscall_retval on amd64

I agree, it would be great to avoid PCB_FULL_IRET, however we should
follow Linux system call ABI.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40152
MFC after: 1 month

(cherry picked from commit a99b890ecd26131e0c91e5693b55f5700c40a847)