os, syscall: remove fallback to pipe syscall on Linux

os, syscall: remove fallback to pipe syscall on Linux

The minimum required Linux kernel version for Go 1.18 will be changed to
2.6.32, see #45964. The pipe2 syscall was added in 2.6.27, so the
fallback to use the pipe syscall in os.Pipe and syscall.forkExecPipe on
Linux can be removed.

For #45964

Change-Id: I033a534f2b39e9bafc9980c9ce980e92f1e3a136
Reviewed-on: https://go-review.googlesource.com/c/go/+/346789
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>