mgmt: hawkbit: remove hb_context.status_buffer_size

remove hb_context.status_buffer_size and replace it with
sizeof(hb_context.status_buffer), because hb_context.status_buffer_size
is never set.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>
(cherry picked from commit 1bea938c)