[mlir][python] Replace PythonAttr mappings with downcasting

Since op `Attribute`s are automatically downcasted on access, these mappings
aren't necessary anymore. Instead we just always generate the getters/setters
for attributes even if there isn't a `PythonAttr` mapping.

depends on D154462

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D154468