Remove leftover code in InspectorFrontendHostStub.js after r158821.

'_fileBuffers' property is not used anymore after r158821, so remove this.

BUG=none.

Review URL: https://codereview.chromium.org/103103004

git-svn-id: svn://svn.chromium.org/blink/trunk@163166 bbb929c8-8fbe-4397-9dbb-9b2b20218538