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/belt.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="#6a6a6a"/>
-  <g transform="translate(13,13) scale(2.3125)" fill="none" stroke="#ffffff" stroke-width="2" stroke-linecap="round" stroke-linejoin="round">
-    <rect x="4" y="10" width="24" height="12" rx="3"/><path d="M9 13 12.5 16 9 19"/><path d="M14.5 13 18 16 14.5 19"/><path d="M20 13 23.5 16 20 19"/>
-  </g>
-</svg>