Remove duplicate plugin.schema.json

[puc9]

Remove ./schema/plugin.schema.json and update VSCode config to use the identical ./validator/plugin.schema.json
This is to avoid having to keep up to date two schema files.

[DogmaDragon]

Technically they are not identical.

https://github.com/stashapp/CommunityScripts/blob/main/schema/plugin.schema.json uses comments which VSCode supports when standard JSON doesn't. See https://code.visualstudio.com/Docs/languages/json#_json-with-comments

That said, I'm not sure it justifies keeping both of the them.

[puc9]

I don't think the comments justify it.
At best, I could move the comments into description/markdownDescription in the schema but I am not sure if it is worth it or not. Adding examples/snippets might be more useful, but the schema is not that complicated to justify the maintenance burden, IMHO.

Also, the comments where originally a helper for me when I wrote the schema, and I'm not 100% sure if I meant to leave them in the schema or I just forgot to remove them before checking in...