MINOR: buffer: replace bi_del() and bo_del() with b_del()

Till now the callers had to know which one to call for specific use cases.
Let's fuse them now since a single one will remain after the API migration.
Given that bi_del() may only be used where o==0, just combine the two tests
by first removing output data then only input.
diff --git a/include/proto/channel.h b/include/proto/channel.h
index 9568050..fdbcc94 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -727,8 +727,7 @@
  */
 static inline void co_skip(struct channel *chn, int len)
 {
-	chn->buf->o -= len;
-
+	b_del(chn->buf, len);
 	if (buffer_empty(chn->buf))
 		chn->buf->p = chn->buf->data;