Documentation/admin: Remove the vsyscall=native documentation

The vsyscall=native feature is gone -- remove the docs.

Fixes: 076ca27 ("x86/vsyscall/64: Drop "native" vsyscalls")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Cc: Borislav Petkov <bp@alien8.de>
Cc: Kernel Hardening <kernel-hardening@lists.openwall.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/d77c7105eb4c57c1a95a95b6a5b8ba194a18e764.1561610354.git.luto@kernel.org

Documentation/admin-guide/kernel-parameters.txt

@@ -5102,12 +5102,6 @@
 			emulate     [default] Vsyscalls turn into traps and are
 			            emulated reasonably safely.
 
-			native      Vsyscalls are native syscall instructions.
-			            This is a little bit faster than trapping
-			            and makes a few dynamic recompilers work
-			            better than they would in emulation mode.
-			            It also makes exploits much easier to write.
-
 			none        Vsyscalls don't work at all.  This makes
 			            them quite hard to use for exploits but
 			            might break your system.