CLEANUP: htx: remove comments about "must be < 256 MB"

Since commit "BUG/MINOR: config: reject configs using HTTP with bufsize
>= 256 MB" we are now sure that it's not possible anymore to have an HTX
block of a size 256 MB or more, even after concatenation thanks to the
tests for len >= htx_free_data_space(). Let's remove these now obsolete
comments.

A BUG_ON() was added in htx_add_blk() to track any such exception if
the conditions would change later, to complete the one that is performed
on the start address that must remain within the buffer.

include/haproxy/htx.h

@@ -439,7 +439,6 @@ static inline struct htx_sl *htx_add_stline(struct htx *htx, enum htx_blk_type t
 
 	size = sizeof(*sl) + p1.len + p2.len + p3.len;
 
-	/* FIXME: check size (< 256MB) */
 	blk = htx_add_blk(htx, type, size);
 	if (!blk)
 		return NULL;