Remove BC symbols from `iota/transaction/__init__.py`

[todofixthis]

Remove the following symbols from iota/transaction/__init__.py:

# Import symbols to package namespace, for backwards-compatibility with
# PyOTA 1.1.x.
from .base import *
from .creation import *
from .types import *
from .utils import *
from .validator import *

As the comment indicates, these symbols are included for compatibility with the 1.1.x branch, which was discontinued nearly a year ago.