BUG/MINOR: quic: Do not ack when probing

BUG/MINOR: quic: Do not ack when probing

<force_ack> boolean variable passed to qc_do_build_pkt() which builds a clear
packet is there to force this function to build an ACK frame regardless of
others conditions. This is used during handshake, when we acknowledge every
handshake packets received.

This variable was already taken into an account by the local variable <must_ack>
which is there at least to ignore any other conditions than this one: "are
we building a probing packet?". Indeed we do not want to add ACK frames when
we probe the peers. This is to have more chances to embed the new duplicated frames
into another packets without splitting them. So, the test on <force_ack> boolean
value is useless, silly and brakes the rule which consists in not acknowledging
when probing.

Must be backported to 2.6.