[mlir][python] Remove __str__ from bindings of StringAttr.

This reverts a feature introduced in commit
2a5d4974. The goal of that commit was to
allow `StringAttr`s to by used transparently wherever Python `str`s are
expected. But, as the tests in https://reviews.llvm.org/D159182 reveal,
pybind11 doesn't do this conversion based on `__str__` automatically,
unlike for the other types introduced in the commit above. At the same
time, changing `__str__` breaks the symmetry with other attributes of
`print(attr)` printing the assembly of the attribute, so the change
probably has more disadvantages than advantages.

Reviewed By: springerm, rkayaith

Differential Revision: https://reviews.llvm.org/D159255