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>

include/net/6lowpan.h

@@ -154,17 +154,6 @@
 #define LOWPAN_FRAG1_HEAD_SIZE	0x4
 #define LOWPAN_FRAGN_HEAD_SIZE	0x5
 
-/*
- * According IEEE802.15.4 standard:
- *   - MTU is 127 octets
- *   - maximum MHR size is 37 octets
- *   - MFR size is 2 octets
- *
- * so minimal payload size that we may guarantee is:
- *   MTU - MHR - MFR = 88 octets
- */
-#define LOWPAN_FRAG_SIZE	88
-
 /*
  * Values of fields within the IPHC encoding first byte
  * (C stands for compressed and I for inline)