latexconf: cond. prepend for backward portability

Removes the automatic prepending of a 'sphinx' namespacing to the configured
class. This way class names need to be written out completely, but users are
also flexible in their choices, e.g. they can choose to use standard LaTeX
document classes like 'book' or 'report'. This enables them to use Sphinx
even for other usecases than documentation, e.g. writing scientific papers.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>