fix: Closing skip logic for some unit tests in slate history section (#5872)

packages/slate-history/test/undo/delete_backward/custom-prop.tsx

@@ -18,5 +18,4 @@ export const input = (
     </block>
   </editor>
 )
-export const skip = true // TODO: see https://github.com/ianstormtaylor/slate/pull/4188
 export const output = cloneDeep(input)

packages/slate-history/test/undo/delete_backward/inline-across.tsx

@@ -26,5 +26,4 @@ export const input = (
     </block>
   </editor>
 )
-export const skip = true // TODO: see https://github.com/ianstormtaylor/slate/pull/4188
 export const output = cloneDeep(input)

packages/slate-history/test/undo/insert_fragment/basic.tsx

@@ -35,4 +35,3 @@ export const input = (
   </editor>
 )
 export const output = cloneDeep(input)
-export const skip = true