Remove the workaround applied to the OptionalOnlyPermission type empty enum entry

[rpl]

As part of #5551 we are injecting the "userScripts" permission into the empty enum entry being imported from the Firefox WebExtensions JSONSchema, to fix an issue with ajv considering the empty enum entry as invalid JSONSchema.

~~Bug 1942814 - Look into alternatives to the empty enum entry part of the OptionalOnlyPermission API schema is tracking looking into an alternative JSONSchema definition to be used on the Firefox side, while this issue is tracking removing the workaround along importing Firefox JSONSchema data that would not need the workaround anymore.~~

We have decided to keep the JSONSchema unchanged on the Firefox side, and instead aim to remove the workaround currently applied to the OptionalOnlyPermission through the changes to the API JSONSchema import tracked by #4511 (merging all per-API JSONSchema files into a single per-"manifest version" JSONSchema file)