MINOR: buffer: remove the check for output on b_del()

b_del() is used in :
  - mux_h2 with the demux buffer : always processes input data
  - checks with output data though output is not considered at all there
  - b_eat() which is not used anywhere
  - co_skip() where the len is always <= output

Thus the distinction for output data is not needed anymore and the
decrement can be made inconditionally in co_skip().
diff --git a/include/common/buf.h b/include/common/buf.h
index 02a66b3..6eec422 100644
--- a/include/common/buf.h
+++ b/include/common/buf.h
@@ -396,10 +396,6 @@
  */
 static inline void b_del(struct buffer *b, size_t del)
 {
-	if (del >= b->output)
-		b->output = 0;
-	else
-		b->output -= del;
 	b->len  -= del;
 	b->head += del;
 	if (b->head >= b->size)
diff --git a/include/proto/channel.h b/include/proto/channel.h
index 291bb08..6241237 100644
--- a/include/proto/channel.h
+++ b/include/proto/channel.h
@@ -767,6 +767,7 @@
 static inline void co_skip(struct channel *chn, int len)
 {
 	b_del(chn->buf, len);
+	chn->buf->output -= len;
 	c_realign_if_empty(chn);
 
 	/* notify that some data was written to the SI from the buffer */