posix: options: don't imply `POSIX_MESSAGE_PASSING`

Don't `imply POSIX_MESSAGE_PASSING` when `POSIX_API=y` as this option
has a non-trivial RAM implication in `HEAP_MEM_POOL_ADD_SIZE_MQUEUE`.

The `mq_*` API is minimally used in-tree, with all users already
enabling the symbol directly.

Signed-off-by: Jordan Yates <jordan@embeint.com>