Remove app manifest generation from Kiota

[maisarissi]

Based on our internal conversation, we have agreed on removing the app manifest generation in Kiota, as this responsibility will be delegated to Teams Toolkit, which is the tool expert on it.
This decision was made based on the decision that Kiota shouldn't be the entry point for folks trying Copilot extensibility. The CTA (call to action) for devs is to start either from TTK or Copilot Studio.

Kiota will remain the expert in handling OpenAPI descriptions, slicing and generating the plugin manifest.

Expected

From now on, when generating a plugin in Kiota, we should get only the plugin manifest and the sliced OpenAPI description.

[andrueastman]

This would involve undoing changes at

• Adding manifest generation as part of plugin generation #4677 - Spec updates
• Adds plugin node in the manifest.json file if it already exists and create the manifest.json when it doesn't #4914 - Manifest generation
• Generates default icons when not present #5025 - Icon generation