types(reactive): remove unnecessary type assertion

[zqran]

Remove unnecessary types in toReadonly function to be consistent with toReactive function.

Details:

export const toReadonly = <T extends unknown>(value: T): T =>
  isObject(value) ? readonly(value as Record<any, any>) : value

when isObject(value) is true, the type of value is Record<any, any>, so no longer need as Record<any, any>

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[haoqunjiang]

Closing as it's been refactored in #8046
Somehow GitHub failed to update the diff in time.