doc: remove stale comment about arm64 port

Fixes #47079.

Change-Id: I8a671e3fdc13083e44e8d89064a5e7621e53bc4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/333075
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

Author: mknyszek

doc/asm.html

@@ -827,10 +827,6 @@ <h3 id="arm">ARM</h3>
 
 <h3 id="arm64">ARM64</h3>
 
-<p>
-The ARM64 port is in an experimental state.
-</p>
-
 <p>
 <code>R18</code> is the "platform register", reserved on the Apple platform.
 To prevent accidental misuse, the register is named <code>R18_PLATFORM</code>.