Fix comments in execTuples.c

Hi hackers,

```
diff --git a/src/backend/executor/execTuples.c b/src/backend/executor/execTuples.c
index 8e02d68824f..b77e9513e7e 100644
--- a/src/backend/executor/execTuples.c
+++ b/src/backend/executor/execTuples.c
@@ -1532,9 +1532,6 @@ ExecSetSlotDescriptor(TupleTableSlot *slot, /* slot to change */
  * and let the upper-level table slot assume ownership of the copy!
  *
  * Return value is just the passed-in slot pointer.
- *
- * If the target slot is not guaranteed to be TTSOpsHeapTuple type slot, use
- * the, more expensive, ExecForceStoreHeapTuple().
  * --------------------------------
  */
 TupleTableSlot *
@@ -1572,9 +1569,6 @@ ExecStoreHeapTuple(HeapTuple tuple,
  * slot is cleared, so that the tuple won't go away on us.
  *
  * Return value is just the passed-in slot pointer.
- *
- * If the target slot is not guaranteed to be TTSOpsBufferHeapTuple type slot,
- * use the, more expensive, ExecForceStoreHeapTuple().
  * --------------------------------
  */
 TupleTableSlot *
@@ -1627,9 +1621,6 @@ ExecStorePinnedBufferHeapTuple(HeapTuple tuple,
 
 /*
  * Store a minimal tuple into TTSOpsMinimalTuple type slot.
- *
- * If the target slot is not guaranteed to be TTSOpsMinimalTuple type slot,
- * use the, more expensive, ExecForceStoreMinimalTuple().
  */
 TupleTableSlot *
 ExecStoreMinimalTuple(MinimalTuple mtup,
```

All of these are reporting an error rather than using ExecForceStore*(), so I just remove these comments.

Thoughts?

--
Regards,
ChangAo Chen