fix: remove non-existent buildsphere tool from dotnet-tools.json

The buildsphere package does not exist on NuGet, causing Renovate
package lookup failures (fixes #140).

Author: phmatray

.config/dotnet-tools.json

@@ -1,13 +1,5 @@
 {
   "version": 1,
   "isRoot": true,
-  "tools": {
-    "buildsphere": {
-      "version": "0.4.2",
-      "commands": [
-        "build-sphere"
-      ],
-      "rollForward": false
-    }
-  }
-}
+  "tools": {}
+}