wstream/shell: Fix memory errors caused by os_system

The os_system function uses a write callback to close the input stream when the
write completes, but this causes a memory error because the callback is invoked
right before the stream is freed by the caller.

This fixes the problem by removing the callback set by os_system. Instead, it
calls job_close_in immediately after writing(the stream will only close after
the write completes). The 'pending' parameter was also removed from the
'write_cb' as it should be hidden by the wstream module.

While the `wstream_set_write_cb` and `job_write_cb` are no longer used, they
will remain in the codebase for future use.