sync/atomic: remove panic64

The func has been unused since https://golang.org/cl/93637 in 2018.

Change-Id: I1cab6f265aa5058ac080fd7c7cbf0fe85370f073
Reviewed-on: https://go-review.googlesource.com/c/go/+/224077
Run-TryBot: Daniel Martí <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Matt Layher <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: mvdan

src/sync/atomic/doc.go

@@ -143,8 +143,3 @@ func StoreUintptr(addr *uintptr, val uintptr)
 
 // StorePointer atomically stores val into *addr.
 func StorePointer(addr *unsafe.Pointer, val unsafe.Pointer)
-
-// Helper for ARM.  Linker will discard on other systems
-func panic64() {
-	panic("sync/atomic: broken 64-bit atomic operations (buggy QEMU)")
-}