[mlir] Remove ``dataclasses`` package from mlir ``requirements.txt`` (#87223)

The ``dataclasses`` package makes sense for Python 3.6, becauses
``dataclasses`` is only included in the standard library with 3.7
version. Now, 3.6 has reached EOL, so all current supported versions of
Python (3.8, 3.9, 3.10, 3.11, 3.12) have this feature in their standard
libraries.

Therefore there's no need to install the ``dataclasses`` package now.