cmd/internal/obj/s390x: delete unused REGZERO constant

When we switched to SSA R0 was made allocatable and no longer holds
zero on s390x.

Change-Id: I1c752bb02da35462a535492379345fa9f4e12cb0
Reviewed-on: https://go-review.googlesource.com/41079Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/internal/obj/s390x/a.out.go

@@ -138,7 +138,6 @@ const (
 
 	REG_RESERVED // end of allocated registers
 
-	REGZERO = REG_R0  // set to zero
 	REGARG  = -1      // -1 disables passing the first argument in register
 	REGRT1  = REG_R3  // used during zeroing of the stack - not reserved
 	REGRT2  = REG_R4  // used during zeroing of the stack - not reserved