Remove unnecessary expectation entry because the test has expected text file

The layout tests reports:
external/wpt/shadow-dom/slots-fallback-in-document.html passed unexpectedly

Note that this CL is not directly related to https://codereview.chromium.org/2842263004.

Review-Url: https://codereview.chromium.org/2887473003
Cr-Commit-Position: refs/heads/master@{#472310}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2775,7 +2775,6 @@ crbug.com/626703 external/wpt/fullscreen/api/element-request-fullscreen-two-ifra
 crbug.com/666703 external/wpt/html/browsers/sandboxing/sandbox-disallow-same-origin.html [ Timeout ]
 crbug.com/626703 external/wpt/html/dom/documents/dom-tree-accessors/Document.currentScript.html [ Timeout ]
 crbug.com/626703 external/wpt/html/browsers/history/the-location-interface/location-protocol-setter-non-broken.html [ Failure Pass ]
-crbug.com/626703 external/wpt/shadow-dom/slots-fallback-in-document.html [ Failure ]
 
 crbug.com/655458 external/wpt/workers/constructors/SharedWorker/undefined-arguments.html [ Failure ]
 crbug.com/655458 external/wpt/workers/baseurl/alpha/worker.html [ Failure ]