CLEANUP: dynbuf: remove the unused b_alloc_fast() function

It is never used anymore since 1.7 where it was used by b_alloc_margin()
then replaced by direct calls to the pools function, and it maintains a
dependency on the exposed pools functions. It's time to get rid of it,
as it's not even certain it still works.
diff --git a/doc/internals/buffer-api.txt b/doc/internals/buffer-api.txt
index ccace49..ac35300 100644
--- a/doc/internals/buffer-api.txt
+++ b/doc/internals/buffer-api.txt
@@ -554,18 +554,6 @@
                     |                  | The allocated buffer is returned, or
                     |                  | NULL in case no memory is available
 --------------------+------------------+---------------------------------------
-b_alloc_fast        | buffer *buf      | allocates a buffer and assigns it to
-                    | ret: buffer *    | *buf. If no memory is available, (1)
-                    |                  | is assigned instead with a zero size.
-                    |                  | No control is made to check if *buf
-                    |                  | already pointed to another buffer. The
-                    |                  | allocated buffer is returned, or NULL
-                    |                  | in case no memory is available. The
-                    |                  | difference with b_alloc() is that this
-                    |                  | function only picks from the pool and
-                    |                  | never calls malloc(), so it can fail
-                    |                  | even if some memory is available
---------------------+------------------+---------------------------------------
 __b_free            | buffer *buf      | releases <buf> which must be allocated
                     | ret: void        | and marks it empty
 --------------------+------------------+---------------------------------------
diff --git a/include/haproxy/dynbuf.h b/include/haproxy/dynbuf.h
index 6835cd6..2a90515 100644
--- a/include/haproxy/dynbuf.h
+++ b/include/haproxy/dynbuf.h
@@ -79,27 +79,6 @@
 	return buf;
 }
 
-/* Allocates a buffer and assigns it to *buf. If no memory is available,
- * ((char *)1) is assigned instead with a zero size. No control is made to
- * check if *buf already pointed to another buffer. The allocated buffer is
- * returned, or NULL in case no memory is available. The difference with
- * b_alloc() is that this function only picks from the pool and never calls
- * malloc(), so it can fail even if some memory is available.
- */
-static inline struct buffer *b_alloc_fast(struct buffer *buf)
-{
-	char *area;
-
-	*buf = BUF_WANTED;
-	area = pool_get_first(pool_head_buffer);
-	if (unlikely(!area))
-		return NULL;
-
-	buf->area = area;
-	buf->size = pool_head_buffer->size;
-	return buf;
-}
-
 /* Releases buffer <buf> (no check of emptiness). The buffer's head is marked
  * empty.
  */