netfilter: nat: remove nf_nat_l3proto.h and nf_nat_core.h

The l3proto name is gone, its header file is the last trace.
While at it, also remove nf_nat_core.h, its very small and all users
include nf_nat.h too.

before:
   text    data     bss     dec     hex filename
  22948    1612    4136   28696    7018 nf_nat.ko

after removal of l3proto register/unregister functions:
   text	   data	    bss	    dec	    hex	filename
  22196	   1516	   4136	  27848	   6cc8 nf_nat.ko

checkpatch complains about overly long lines, but line breaks
do not make things more readable and the line length gets smaller
here, not larger.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>