[mathml] Remove 'lengths-2.html' from test expectation file

Remove 'lengths-2.html' from test expectation file since
it started passing recently.

Bug: 6606
Change-Id: Ib3a849f26fda7d343eee66106801bf8dc828961f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359037
Commit-Queue: Rob Buis <rbuis@igalia.com>
Reviewed-by: Frédéric Wang <fwang@igalia.com>
Cr-Commit-Position: refs/heads/master@{#798737}

third_party/blink/web_tests/TestExpectations

@@ -1339,7 +1339,6 @@ crbug.com/6606 external/wpt/mathml/relations/css-styling/displaystyle-015.html [
 crbug.com/6606 external/wpt/mathml/relations/css-styling/displaystyle-1.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/relations/css-styling/displaystyle-2.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/relations/css-styling/ignored-properties-001.html [ Failure Timeout ]
-crbug.com/6606 external/wpt/mathml/relations/css-styling/lengths-2.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/relations/css-styling/mathvariant-bold-fraktur.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/relations/css-styling/mathvariant-bold-italic.html [ Failure ]
 crbug.com/6606 external/wpt/mathml/relations/css-styling/mathvariant-bold-sans-serif.html [ Failure ]