unix: drop fallback to pipe in Pipe on linux/arm

Same as CL 347349 does for package syscall.

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

For golang/go#45964

Change-Id: I8e222dc4b373b22f91bc1ddcc48f58a37dafcffa
Reviewed-on: https://go-review.googlesource.com/c/sys/+/347331
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/unix/syscall_linux_arm.go b/unix/syscall_linux_arm.go
index b961a62..31e029b 100644
--- a/unix/syscall_linux_arm.go
+++ b/unix/syscall_linux_arm.go
@@ -26,11 +26,7 @@
 		return EINVAL
 	var pp [2]_C_int
-	// Try pipe2 first for Android O, then try pipe for kernel 2.6.23.
 	err = pipe2(&pp, 0)
-	if err == ENOSYS {
-		err = pipe(&pp)
-	}
 	p[0] = int(pp[0])
 	p[1] = int(pp[1])