move ctor for strict_not_null should be removed

[beinhaerter]

In commit 9ff6e19 (issue #743) in 2018 strict_not_null was introduced. The interface was almost identical to the one of not_null, but the constructor is explicit.

Later in commit 6b23937 in 2020 the move ctor for not_null was removed, but the move constructor for strict_not_null was not removed.

In my opinion the move constructor for strict_not_null should be removed as it was done with the move constructor for not_null. This aligns the interfaces of both classes to be the same. And it removes code that looks like it moves but instead copies, which is at least surprising.

[beinhaerter]

Thanks to @maflcko who brought my attention to it in #1245.

[maflcko]

(off-topic: Heh, I still prefer #1238 to make it movable, but this seems possibly controversial and also off-topic here)

on-topic: I agree that removing it is the least confusing and most self-consistent fix for now.