Prompt Fragment view reset should only reset built-ins

[sgraband]

Bug Description:

Currently when resetting all prompt fragments from the view (to built-ins) it also reset prompts that do not have a built-in removing them all together, which is not what should happen. We should simply remove all customizations for fragments that have a built-in.

Steps to Reproduce:

1. Reset all prompts
2. Notice hat the project-info.prompttempalte is removed.

[JonasHelming]

@ndoschek I find this pretty important!