[mlir][python] Remove "Raw" OpView classes

The raw `OpView` classes are used to bypass the constructors of `OpView`
subclasses, but having a separate class can create some confusing
behaviour, e.g.:
```
op = MyOp(...)
# fails, lhs is 'MyOp', rhs is '_MyOp'
assert type(op) == type(op.operation.opview)
```

Instead we can use `__new__` to achieve the same thing without a
separate class:
```
my_op = MyOp.__new__(MyOp)
OpView.__init__(my_op, op)
```

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D143830