math,net: omit explicit true tag expr in switch

math,net: omit explicit true tag expr in switch

Performed `switch true {}` => `switch {}` replacement.

Found using https://go-critic.github.io/overview.html#switchTrue-ref

Change-Id: Ib39ea98531651966a5a56b7bd729b46e4eeb7f7c
Reviewed-on: https://go-review.googlesource.com/123378
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>