6lowpan: remove unused LOWPAN_FRAG_SIZE define

This define is unused since commit
96cb3eb7 ("6lowpan: fix fragmentation on
sending side"). It is a worst case scenario for payload calculation.
Since commit 96cb3eb7

 we calculation the
payload to use the optimal size.

This define is also necessary for ieee802154 6lowpan only and the file
include/net/6lowpan.h should contain generic 6lowpan things only.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>