os: remove dependency on strings package

Historically the os package has not imported the strings package.
That was enforced by go/build.TestDependencies, but that test
was accidentally broken (#43249). A dependency of os on strings
was accidentally added by CL 266364; remove it.

For #42026
For #43249

Change-Id: If932308f30561fdcc5c608d7563e849c0d2870d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/279072


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>