Remove unused ARG arch

This is unused since #1804

Author: webmaster128

Dockerfile

@@ -1,7 +1,6 @@
 # docker build . -t cosmwasm/wasmd:latest
 # docker run --rm -it cosmwasm/wasmd:latest /bin/sh
 FROM golang:1.21-alpine3.17 AS go-builder
-ARG arch=x86_64
 
 # this comes from standard alpine nightly file
 #  https://github.com/rust-lang/docker-rust-nightly/blob/master/alpine3.12/Dockerfile