net/http: fix ServeMux pattern registration

net/http: fix ServeMux pattern registration

When the httpmuxgo121 GODEBUG setting was active, we were registering
patterns in the old and the new way. Fix to register only in the old
way.

Change-Id: Ibc1fd41e7f4d162ee5bc34575df409e1db5657cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/533095
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Olena Synenka <olenasynenka@google.com>
Reviewed-by: Damien Neil <dneil@google.com>