Remove layout_2020::display_list, use WebRender display lists instead

[SimonSapin]

This change is 

[highfive]

Heads up! This PR modifies the following files:

• @emilio: components/layout/display_list/items.rs, components/layout/display_list/webrender_helpers.rs, components/layout/display_list/mod.rs

[highfive]

Warning

• These commits modify layout code, but no tests are modified. Please consider adding a test!

[nox]

@bors-servo r+

[bors-servo]

📌 Commit c901f68 has been approved by nox

[jdm]

@bors-servo r-

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[SimonSapin]

Made the unit test to pass, though it’s arguably less meaningful now.

[nox]

@bors-servo r+

[bors-servo]

📌 Commit 3e6489f has been approved by nox

[bors-servo]

⌛ Testing commit 3e6489f with merge c2178ff...

[bors-servo]

💔 Test failed - status-taskcluster

[CYBAI]

@bors-servo retry

• #23290

[bors-servo]

💣 Failed to start rebuilding: Unknown error

[bors-servo]

⌛ Testing commit 3e6489f with merge 5bcb1b5...

[bors-servo]

☀️ Test successful - linux-rel-css, linux-rel-wpt, status-taskcluster
Approved by: nox
Pushing 5bcb1b5 to master...