Remove struct arpcom. It is unused by most interface types, that allocate
it, except Ethernet, where it carried ng_ether(4) pointer.
For now carry the pointer in if_l2com directly.

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git-svn-id: svn+ssh://svn.freebsd.org/base/head@274231 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f