runtime/internal/atomic: remove erroneous ABI wrappers

CL 179862 introduced go:linkname directives to create ABI wrappers for
Store and Store64 on s390x, but a concurrent change (CL 180439)
replaced the Go definitions of these functions with assembly
definitions. This resulted in conflicting definitions for the ABI0
symbols, which led to a bootstrap linking failure.

Fix this by removing the now-incorrect go:linkname directives for
Store and Store64. This should fix the linux-s390x builders.

Updates #31230.

Change-Id: I8de8c03c23412fc217d428c0018cc56eb2f9996f
Reviewed-on: https://go-review.googlesource.com/c/go/+/181078Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/runtime/internal/atomic/atomic_s390x.go

@@ -10,8 +10,6 @@ import "unsafe"
 //go:linkname Load
 //go:linkname Loadp
 //go:linkname Load64
-//go:linkname Store
-//go:linkname Store64
 
 //go:nosplit
 //go:noinline