[arm64] Remove unused variable in debug-arm64.cc.

BUG=

Change-Id: Ibf49df830153a829015723826dacc6939fb42189
Reviewed-on: https://chromium-review.googlesource.com/452377Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43689}

src/debug/arm64/debug-arm64.cc

@@ -84,7 +84,6 @@ bool DebugCodegen::DebugBreakSlotIsPatched(Address pc) {
 void DebugCodegen::GenerateDebugBreakStub(MacroAssembler* masm,
                                           DebugBreakCallHelperMode mode) {
   __ RecordComment("Debug break");
-  Register scratch = x10;
   {
     FrameScope scope(masm, StackFrame::INTERNAL);