Revert "os: check for valid Windows path when creating files"

Revert "os: check for valid Windows path when creating files"

This reverts commit CL 618496.

Reason for revert: https://github.com/golang/go/issues/54040#issuecomment-2485151973

Change-Id: I3bf27f7fdd475a005cb6aa190994153504e96fb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/629595
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>