net: icmpv6: Remove unnecessary cache pointers for src and dst

Earlier net_nbuf_copy_all() mangled the original buffer. So cacheing
src and dst address and other values was necessary. Now original
buffer does not get affected by net_nbuf_copy_all() call. Cacheing
is not required.

Change-Id: I8a8534f7ec299853b9b296d85e8aacecd3768c8e
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>