fixes #70: remove un-necessary constructors from expression

[tzlaine]

This is unnecessary for what we really want -- to make expression<> constexpr. This has latter change just been committed.

[rgrover]

What I would really like to have is the ability to initialize an expression like an aggregate. With the help of a deduction guide, such as

template <typename T>
expression(T)->expression<expr_kind::terminal, boost::hana::tuple<T>>;

I can then write expression{1}

[tzlaine]

CTAD works with ctors. Define one for expression<> if you like, and construct expression<>s with "expression(1)". Does that not work for some reason?

[rgrover]

yes, CTAD works with constructors. I essentially want constexpr expression. thanks.