Remove unused variables from assembler-arm.cc.

Local arm.debug build with make fails - not sure why arm build doesn't fail on
the waterfall.

R=rmcilroy@chromium.org
BUG=

Review URL: https://codereview.chromium.org/337273003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21964 ce2b1a6d-e550-0410-aec6-3dcde31c8c00