net/http: drop mimesniff for audio/basic

net/http: drop mimesniff for audio/basic

The WHATWG Mime Sniffing Standard we follow dropped support for
.snd / audio/basic.
https://github.com/whatwg/mimesniff/issues/151

Change-Id: Iae1bd8b29063b06b5b3909a944e12ead5974a526
Reviewed-on: https://go-review.googlesource.com/c/go/+/400754
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Auto-Submit: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>