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

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.

(cherry picked from commit 3d5f19e04d88e7c8f71cba4ea12e383c91de89f6)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 86cb2cd3c68a0f3072a326def89449e10760423d)
[wt: no change in htx.h, ctx updates in htx.c]
Signed-off-by: Willy Tarreau <w@1wt.eu>