Remove useless code

`to_unicode` variable was created but was not used.

or other variable-length column type with unbounded length
by assigning the token ``"max"`` to the
length attribute of the string.   While using the ``"max"`` token
explicitly is supported by the SQL Server dialect, it isn't part
of the normal contract of the base string types, and instead the
length should just be left as None.   The dialect now assigns the
length to None on reflection of the type so that the type behaves
normally in other contexts.
fixes #3504

up SQLAlchemy's version detection under Python 3 and inadvertently
not using the correct unicode mode for Python 3.  This would cause
issues such as bound variables mis-interpreted as NULL and rows
silently not being returned.
fixes #3491