wireless:mac80211: kill unuse macro MESH_CFG_CMP_LEN in mesh.h

Commit 00d3f14c has removed the references of this macro,
but left it only. So remove this definition.

commit 00d3f14c
Author: Johannes Berg <johannes@sipsolutions.net>
Date:   Tue Feb 10 21:26:00 2009 +0100

    mac80211: use cfg80211s BSS infrastructure

    Remove all the code from mac80211 to keep track of BSSes
    and use the cfg80211-provided code completely.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>