net: ipv6: Remove invalid/unneeded code from fragmentation logic

Skipping both next_hdr_off and last_hdr_off and filling last_hdr
variable doesn't make much sense, as this effectively moves the packet
cursor inside/behind the last (L4) header with no particular meaning.
Plus the last_hdr variable isn't really used anywhere, which kind of
proves the point. Therefore, remove the unused variable and needless
net_pkt operations.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>