Remove unused ui::AXPlatformNode pointer from BrowserAccessibility.

A while ago, we thought we would keep the the platform node as a seperate
object.  Most of this was removed in
https://codereview.chromium.org/2886353003 but we missed this.

BUG=703369

Review-Url: https://codereview.chromium.org/2930413004
Cr-Commit-Position: refs/heads/master@{#479644}
1 file changed