1871154 - Remove dead code about absolute positioned subgrids

Status: NEW

(Core :: Layout: Grid, task)

Description

[Daniel Holbert [:dholbert]]

As of bug 1800563, it's no longer possible to have absolutely-positioned subgrids. Previously, these were something that nsGridContainerFrame attempted to accommodate.

There's likely some code that's unnecessary now that this situation is impossible. e.g.:
https://searchfox.org/mozilla-central/rev/3bd65516eb9b3a9568806d846ba8c81a9402a885/layout/generic/nsGridContainerFrame.cpp#959-960

// the same axis.  It may have zero tracks (a grid with only abs.pos.
// subgrids/items may have zero tracks).

https://searchfox.org/mozilla-central/rev/3bd65516eb9b3a9568806d846ba8c81a9402a885/layout/generic/nsGridContainerFrame.cpp#4531-4532

// Abs.pos. subgrids may have kAutoLine in their area.  Map those to the edge
// line in the parent's grid (zero-based line numbers).

Filing this bug on the followup cleanup work of removing/reconciling this code with the fact that abs.pos. subgrids are now impossible (they just ignore their subgrid styling).