6lowpan: remove unused LOWPAN_FRAG_SIZE define

6lowpan: remove unused LOWPAN_FRAG_SIZE define

This define is unused since commit
852358846db5deaf8507b5a1311921aa10c9176b ("6lowpan: fix fragmentation on
sending side"). It is a worst case scenario for payload calculation.
Since commit 852358846db5deaf8507b5a1311921aa10c9176b 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>