Undo build button grid icons

Remove the per-building SVG icons and all their wiring (the icon assets,
the QtSvg-based icon loading in BuildButtonGrid, the icon dir plumbing in
MainWindow, and the Qt5::Svg dependency), leaving only the Demolish ->
Deconstruct rename from the previous commit.

This is the inverse of the icon half of the previous commit, paired with a
following revert that re-adds the icons. Cherry-picking the previous
"rename + icons" commit together with this undo reconstructs the rename on
its own.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

bin/app/data/icons/buildings/splitter.svg

@@ -1,6 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100" width="100" height="100">
-  <rect width="100" height="100" rx="22" fill="#7a7a5a"/>
-  <g transform="translate(13,13) scale(2.3125)" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
-    <path d="M5 16h11"/><path d="M16 8v16"/><path d="M13.5 10.5 16 8 18.5 10.5"/><path d="M13.5 21.5 16 24 18.5 21.5"/>
-  </g>
-</svg>