assets: drop the Transition placeholder icon (fix develop CI)

#519 delivered tet-hex transitions through mesh.cells and removed the
'coming soon' mesh.transition catalogue entry, leaving its icon without an
entry (icons::tests::every_catalogue_entry_has_an_icon_and_every_icon_an_entry).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

assets/icons/mesh.transition.svg

@@ -1,10 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="24" height="24" viewBox="0 0 24 24">
-  <title>mesh.transition</title>
-  <g id="Layer_1" data-name="Layer 1">
-    <path d="M3 4 L21 4 L21 20 L3 20 Z" fill="#2d4e70" stroke="#dde2ea" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round"/>
-    <path d="M7.5 4 L7.5 20 M3 9.33 L12 9.33 M3 14.67 L12 14.67" fill="none" stroke="#dde2ea" stroke-linecap="round" stroke-linejoin="round"/>
-    <path d="M12 4 L21 9.33 L12 14.67 L21 20 M12 14.67 L21 14.67 M12 4 L21 4" fill="none" stroke="#dde2ea" stroke-linecap="round" stroke-linejoin="round"/>
-    <path d="M12 4 L12 20" fill="none" stroke="#4f9ce8" stroke-width="1.75" stroke-linecap="round" stroke-linejoin="round"/>
-  </g>
-</svg>

assets/icons/src/mesh.transition.json

@@ -1 +0,0 @@
-{"shapes": [{"rect": [3, 4, 18, 16], "fill": "soft", "stroke": "line"}, {"d": "M7.5 4 L7.5 20 M3 9.33 L12 9.33 M3 14.67 L12 14.67", "stroke": "line", "width": 1}, {"d": "M12 4 L21 9.33 L12 14.67 L21 20 M12 14.67 L21 14.67 M12 4 L21 4", "stroke": "line", "width": 1}, {"d": "M12 4 L12 20", "stroke": "accent", "width": 1.75}]}