Remove a redundant comparison that raises an exception in Python 3

sphinx.__version__ is a string, 1.1 is a float: the comparison always
returns True in Python 2. In Python 3 however the

> <, <=, > and >= operators will raise a TypeError exception when
> comparing a complex number with another built-in numeric type, when
> the objects are of different types that cannot be compared, or in
> other cases where there is no defined ordering

[0]. str and float are such unorderable types.

[0] http://docs.python.org/3.3/library/stdtypes.html#comparisons
Merge pull request #2685 from mineo/doc-conf-version-comp

Remove a redundant comparison that raises an exception in Python 3