BUG/MINOR: quic: Stop removing ACK ranges when building packets

BUG/MINOR: quic: Stop removing ACK ranges when building packets

Since this commit:

    BUG/MINOR: quic: Possible wrapped values used as ACK tree purging limit.

There are more chances that ack ranges may be removed from their trees when
building a packet. It is preferable to impose a limit to these trees. This
will be the subject of the a next commit to come.

For now on, it is sufficient to stop deleting ack range from their trees.
Remove quic_ack_frm_reduce_sz() and quic_rm_last_ack_ranges() which were
there to do that.
Make qc_frm_len() support ACK frames and calls it to ensure an ACK frame
may be added to a packet before building it.

Must be backported to 2.6 and 2.7.