MINOR: quic: remove ->offset qf_crypto struct field

MINOR: quic: remove ->offset qf_crypto struct field

This patch follows this previous bug fix:

    BUG/MINOR: quic: reorder fragmented RX CRYPTO frames by their offsets

where a ebtree node has been added to qf_crypto struct. It has the same
meaning and type as ->offset_node.key field with ->offset_node an eb64tree node.
This patch simply removes ->offset which is no more useful.

This patch should be easily backported as far as 2.6 as the one mentioned above
to ease any further backport to come.