…ariable_win7`.

…ck`.

…iable`.

We never provided user-visible declarations for what they returned. It's extremely unlikely that users were successfully calling them in any useful way, so the bincompat risk of nulling out the vptrs is minimal.

I am opting to `=delete` these functions, instead of simply not declaring them, so that attempted calls will have a slightly more comprehensible error message. As the TRANSITION comments note, in theory we could provide implementations that return the underlying Win32 machinery which actually would be possible to document.

I am also changing `condition_variable`'s `native_handle_type` from `_Cnd_t` (an alias provided by `<xthreads.h>` for `_Cnd_internal_imp_t*`, which is forward-declared and never defined for users) to `void*` and grouping it with the function, which follows the Standard's order.

This reverts commit 34fdc17.