gnomeExtensions.workspace-grid: Remove

workspace-matrix largely replaces workspace-grid [1].

Also, workspace-grid.nix was accidentally dropped from
top-level/all-packages.nix in the Gnome 3.18 -> 3.20 bump in
5a245c2, so this file has been
unreferenced for three years.

[1] zakkak/workspace-grid#120 (comment)