math: remove redundant infinity tests

math: remove redundant infinity tests

These cases are covered by existing comparisons against constants.

Change-Id: I19ad530e95d2437a8617f5229495da591ceb779a
Reviewed-on: https://go-review.googlesource.com/c/go/+/692255
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Sean Liao <sean@liao.dev>