Revert "[mlir] Add baseAttr to TypedArrayAttrBase."

This reverts commit eec36909.

This modeling is incorrect. baseAttr is intended for attribute
decorators that are not backed by C++ attribute classes. It essentially
says DerivedAttr isa BaseAttr, which is wrong for ArrayAttr classes.
If one needs to store the element type, it should be stored as a
separate filed in the tablegen class.