Removed stale @tryghost/errors declaration (#28442)

no ref

This is a types-only change.

`@tryghost/errors` has types. But we claimed that it didn't in this
file! This removes that.

Author: EvanHahn

ghost/core/core/server/api/endpoints/libraries.d.ts

@@ -1,4 +1,3 @@
 // Type declarations for untyped modules used by TypeScript endpoints
 declare module '@tryghost/api-framework';
 declare module '@tryghost/tpl';
-declare module '@tryghost/errors';