fix(contracts): drop the dead preview recording frame schema

Second adversarial pass caught my restore being too broad. #9129 deleted exactly
one symbol here, `DesktopPreviewRecordingFrameSchema`, and it has no consumers.
I restored the whole surrounding block on the strength of the neighbouring
`DesktopPreviewRecordingSource` schema, which upstream never touched and which
apps/desktop does use.

The `DesktopPreviewRecordingFrame` interface stays: it is live in the preload,
the pip preload, and the preview Manager. Only the unused codec goes, and with
it a piece of dead public API, since contracts re-exports ./ipc.ts wholesale.

Author: rynfar

packages/contracts/src/ipc.ts

@@ -693,15 +693,6 @@ export interface DesktopPreviewRecordingFrame {
   receivedAt: string;
 }
 
-export const DesktopPreviewRecordingFrameSchema: Schema.Codec<DesktopPreviewRecordingFrame> =
-  Schema.Struct({
-    tabId: DesktopPreviewTabIdSchema,
-    data: Schema.String,
-    width: Schema.Number,
-    height: Schema.Number,
-    receivedAt: Schema.String,
-  });
-
 export interface DesktopPreviewRecordingSource {
   sourceId: string;
   width: number;