Remove force writing **/*.mts to tsconfig (#83967)
It was added at https://github.com/vercel/next.js/pull/83556. We don't
force writing `**/*.tsx` or `**/*.ts` to tsconfig, but forcing
`**/*.mts` is too harsh, when its intention was to support
`next.config.mts`. Just for the empty tsconfig or empty include was
enough.
