Hotfix: Remove DemoScripts asmdef

The DemoScripts.asmdef was a bit of experimental cruft that snuck in via
my Yamato changes. This causes Demo.cs to compile as an Editor script,
which  means it can no longer be instantiated in the scene hierarchy
(this breaks the demo).  Removing the asmdef puts things back to normal.

Tested by running the demo.

tutorials/pick_and_place/PickAndPlaceProject/Assets/DemoScripts/DemoScripts.asmdef

-{
-    "name": "DemoScripts",
-    "rootNamespace": "",
-    "references": [
-        "GUID:625bfc588fb96c74696858f2c467e978",
-        "GUID:b1ef917f7a8a86a4eb639ec2352edbf8",
-        "GUID:465c1207fffb96245a352265e7622205"
-    ],
-    "includePlatforms": [
-        "Editor"
-    ],
-    "excludePlatforms": [],
-    "allowUnsafeCode": false,
-    "overrideReferences": false,
-    "precompiledReferences": [],
-    "autoReferenced": true,
-    "defineConstraints": [],
-    "versionDefines": [],
-    "noEngineReferences": false
-}

tutorials/pick_and_place/PickAndPlaceProject/Assets/DemoScripts/DemoScripts.asmdef.meta

-fileFormatVersion: 2
-guid: 80312ba658e2b5f41bf853abe51fbe2f
-AssemblyDefinitionImporter:
-  externalObjects: {}
-  userData: 
-  assetBundleName: 
-  assetBundleVariant: