CLEANUP: http_ana: remove unused assignation of `att_beg`

CLEANUP: http_ana: remove unused assignation of `att_beg`

`att_beg` is assigned to `next` at the end of the `for` loop, but is
assigned to `prev` at the beginning of the loop, which is itself
assigned to `next` after each loop. So it represents a double
assignation for the same value. Also `att_beg` is not used after the end
of the loop.

this is a partial fix for github issue #923, all the others could
probably be marked as intentional to protect future changes.

no backport needed.

Signed-off-by: William Dauchy <wdauchy@gmail.com>