[mlir][vector][nfc] Simplify `in_bounds` attr update (#100334)
Since the `in_bounds` attribute is mandatory, there's no need for logic
like this (`readOp.getInBounds()` is guaranteed to return a non-empty
ArrayRef):
```cpp
ArrayAttr inBoundsAttr = readOp.getInBounds()
? rewriter.getArrayAttr( readOp.getInBoundsAttr().getValue().drop_back(dimsToDrop))
: ArrayAttr();
```
Instead, we can do this:
```cpp
ArrayAttr inBoundsAttr = rewriter.getArrayAttr(
readOp.getInBoundsAttr().getValue().drop_back(dimsToDrop));
```
This is a small follow-up for #97049 - this change should've been
included there.