runtime: remove unused armArch

runtime.armArch is unused on linux/arm since CL 93637. The new code in
runtime/internal/atomic (added by CL 111315) only checks runtime.goarm.

Change-Id: Ic097ee6750e39abb20f45770a1c7c2d925f02408
Reviewed-on: https://go-review.googlesource.com/c/140077
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/os_linux_arm.go

@@ -4,20 +4,14 @@
 
 package runtime
 
-import (
-	"internal/cpu"
-	"unsafe"
-)
+import "internal/cpu"
 
 const (
-	_AT_PLATFORM = 15 //  introduced in at least 2.6.11
-
 	_HWCAP_VFP   = 1 << 6  // introduced in at least 2.6.11
 	_HWCAP_VFPv3 = 1 << 13 // introduced in 2.6.30
 )
 
 var randomNumber uint32
-var armArch uint8 = 6 // we default to ARMv6
 
 func checkgoarm() {
 	// On Android, /proc/self/auxv might be unreadable and hwcap won't
@@ -47,12 +41,6 @@ func archauxv(tag, val uintptr) {
 		randomNumber = uint32(startupRandomData[4]) | uint32(startupRandomData[5])<<8 |
 			uint32(startupRandomData[6])<<16 | uint32(startupRandomData[7])<<24
 
-	case _AT_PLATFORM: // v5l, v6l, v7l
-		t := *(*uint8)(unsafe.Pointer(val + 1))
-		if '5' <= t && t <= '7' {
-			armArch = t - '0'
-		}
-
 	case _AT_HWCAP:
 		cpu.HWCap = uint(val)
 	case _AT_HWCAP2: