Remove flaky test entry for 2 accessibility tests.

 * accessibility/element-role-mapping-normal.html
 * accessibility/table-header-column-row.html

The flakiness is due to slowness which is fixed by
https://crrev.com/c/1306965


Bug: 898049,899885
Change-Id: If4a9a770833344b3b55e89bbc3379f3cc9560e0e
Reviewed-on: https://chromium-review.googlesource.com/c/1309375Reviewed-by: Yi Gu <yigu@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604145}

third_party/WebKit/LayoutTests/TestExpectations

@@ -5511,8 +5511,6 @@ crbug.com/895777 external/wpt/infrastructure/assumptions/blank.html [ Failure Cr
 crbug.com/896765 [ Linux Mac Win ] http/tests/performance-timing/element-timing/observe-child-element.html [ Failure Pass ]
 
 #Sheriff 2018-10-22
-crbug.com/897849 [ Mac10.13 Debug ] accessibility/element-role-mapping-normal.html [ Pass Timeout ]
-crbug.com/897849 [ Mac10.13 Debug ] accessibility/table-header-column-row.html [ Pass Timeout ]
 crbug.com/897075 [ Win10 ] inspector-protocol/css/css-coverage-new-stylesheet.js [ Failure Pass ]
 
 #Sheriff 2018-10-23