sys: lib: introduce ringbuffer_remove()

This commit adds a function that deletes n elements from the ringbuffer
without actually writing them somewhere.
```c
if (n >= rb->avail) {
    n = rb->avail;
    rb->avail = 0;
    rb->start = 0;
}
else {
    unsigned new_start = rb->start + n;
    if ((new_start >= rb->size) || (new_start < rb->start)) {
        new_start -= rb->size;
    }
    rb->start = new_start;
    rb->avail -= n;
}
return n;
```