Remove GitNexus logo from the distributable

assets/gitnexus-logo.svg is branding for the Gitea instance that hosts this
project, not for NexusOS itself, so it does not belong in the public repo.
.gitignore already listed it, but gitignore does not apply to tracked files,
so it stayed until removed explicitly.

The README header already points at assets/n-small.png, so nothing references
it anymore.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

assets/gitnexus-logo.svg

@@ -1,14 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="12 3 45 54" fill="none">
-  <g stroke="#7e14ff" stroke-width="9" stroke-linecap="round" stroke-linejoin="round">
-    <!-- trunk -->
-    <path d="M28 13 L28 40"/>
-    <!-- fork -->
-    <path d="M28 29 L47 16"/>
-  </g>
-  <!-- bold wide arrowhead -->
-  <path d="M28 56 L14 37 L42 37 Z" fill="#7e14ff"/>
-  <!-- commit nodes -->
-  <circle cx="28" cy="13" r="8" fill="#7e14ff"/>
-  <circle cx="28" cy="29" r="8" fill="#7e14ff"/>
-  <circle cx="49" cy="15" r="7" fill="#47bfff"/>
-</svg>