Remove unnecessary parenthesis gcc 8 complains about.

[Romain-Geissler-1A]

Hi,

I had new warnings when migrating to gcc 8 (which are blocking for anyone using -Werror).

Cheers,
Romain

[Romain-Geissler-1A]

Here is what I get:

/remote/mw/combld/delivery/pack18.continuous/components.osp/osp/Boost/18-0-0-0/include/boost/mpl/assert.hpp:188:21: error: unnecessary parentheses in declaration of ‘assert_arg’ [-Werror=parentheses]
failed ************ (Pred::************
^
/remote/mw/combld/delivery/pack18.continuous/components.osp/osp/Boost/18-0-0-0/include/boost/mpl/assert.hpp:193:21: error: unnecessary parentheses in declaration of ‘assert_not_arg’ [-Werror=parentheses]
failed ************ (boost::mpl::not_::************

[Romain-Geissler-1A]

Replaced by #34