posix: cond: use k_condvar instead of posix_cond

The internal representation of `pthread_cond_t`,
`struct posix_cond`, is an identical clone of `struct k_condvar`
but without the benefit of being able to use all of the
existing `k_condvar_*()` suite of functions.

The first step in the right direction was switching
the external representation of `pthread_cond_t` to a simple
`int`. Let's take the next step in the right direction, which
is getting rid of `struct posix_cond`.

For now, let's keep this change as a simple type
substitution. Eventually, we should be able to fully switch
to Zephyr API internally.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>