mtu: remove MTU of 1500 bytes

Remove the highest, rare case of 1500 bytes MTU, and mitigate the case
of fragmented packets dropped asymmetrically - IP_NOFRAG unavailable in
PJNATH for the moment -
while still allowing to use a lower MTU on links where the actual MTU is
very low and no packet fragmentation is possible.
In this case the ping packet would be dropped and the correct MTU would
be discovered on both sides.

Change-Id: I1870eb08ad8298aff134995388b4958490426351