x86/entry/64: Remove obsolete comment on tracing vs. SYSRET

[ Upstream commit eb43c9b1 ]

This comment comes from a time when the kernel attempted to use SYSRET
on all returns to userspace, including interrupts and exceptions.  Ever
since commit fffbb5dc

 ("Move opportunistic sysret code to syscall code
path"), SYSRET is only used for returning from system calls. The
specific tracing issue listed in this comment is not possible anymore.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20230721161018.50214-2-brgerst@gmail.com
Stable-dep-of: ac8b270b

 ("x86/bhi: Avoid warning in #DB handler due to BHI mitigation")
Signed-off-by: Sasha Levin <sashal@kernel.org>