Fixed scope exit class.

[5cript]

Addressed the following:

• Move assignment posed the question whether a valid onExit should be called or not. This just erases it, because there is no simple definitive answer.
• ScopeExit is no longer a template, the deduction guide was incorrect to get proper forwarding. The template parameter only for the constructor was unnecessary anyways. Moving it to the constructor only.
• Make it explicit that the class expects a noexcept function. It was that way before by the nature of destructors being noexcept, but it is now more explicit about it.

Found by @DNKpp