[html/canvas] tests for TextMetrics.advances should not be in WPT as this attribute is not in the spec

[jfkthame]

The spec for the TextMetrics interface does not include an advances attribute. This was proposed at some point in the past, but dropped due to lack of clarity on exactly how such an API should work; see https://github.com/whatwg/html/issues/4026.

There are tests for TextMetrics.advances in html/canvas/element/drawing-text-to-the-canvas and html/canvas/offscreen/text, currently shown as passing in Chrome and Edge, but given that TextMetrics.advances was acknowledged to be premature/flawed and was removed from the spec, I don't think these tests are appropriate for WPT at this time.