cmd/gc: fix build - remove unused variables in walkprint
cmd/gc: fix build - remove unused variables in walkprint

TBR=austin
CC=golang-codereviews
https://codereview.appspot.com/162420043

webfd/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh