Stop reducing RA advertised ipv6 mtu by 16 - not needed.

This effectively reverts:

  commit 34d43616
  Author: Maciej Żenczykowski <maze@google.com>
  Date:   Wed Feb 19 01:24:39 2020 -0800

      Reduce advertised ipv6 mtu by 16 to fit ethernet header

      This is a temporary hack to workaround the inability of current
      kernel's ebpf bpf_skb_change_mode() function to prefix a 14-byte
      ethernet header on to a packet without going over the upstream
      (source, rawip) interface's mtu *before* we bpf_redirect() to
      the downstream (destination, ethernet) interface.

      Test: build, atest, atest TetheringTests
      Bug: 149816401

Test: flashed a flame with new kernel and it works at 1500 mtu
Bug: 149816401
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I76a75a16fa27b47d78816b2f9379ef4bb68beb00