[turbolizer][cleanup] Remove focusable attribute. It doesn't exist

Source: https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes

Bug: v8:7327
Change-Id: I2f91b7dc619d70ae29600ae7f304d9944994c863
Notry: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1925151Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65083}

tools/turbolizer/src/graph-view.ts

@@ -71,7 +71,6 @@ export class GraphView extends PhaseView {
     // Listen for key events. Note that the focus handler seems
     // to be important even if it does nothing.
     svg
-      .attr("focusable", false)
       .on("focus", e => { })
       .on("keydown", e => { view.svgKeyDown(); });