Drop bundled copy of linux/if_arp.h

As far as I can see, we use this to get a list of ARPHRD_* defines (used in
particular for Type= in .link files). If we drop our copy, and build against
old kernel headers, the user will have a shorter list of types available.  This
seems OK, and I don't think it's worth carrying our own version of this file
just to have newest possible entries.

7c5b9952 recently updated this file, but we'd
have to update it every time the kernel adds new entries. But if we look at
the failure carefully:

src/basic/arphrd-from-name.gperf:65:16: error: ‘ARPHRD_MCTP’ undeclared (first use in this function); did you mean ‘ARPHRD_FCPP’?
   65 | MCTP, ARPHRD_MCTP
      |                ^~
      |                ARPHRD_FCPP

we see that the list we were generating was from the system headers, so it was
only as good as the system headers anyway, without the newer entries in our
bundled copy, if there were any. So let's make things simpler by always using
system headers.

And if somebody wants to fix things so that we always have the newest list,
then we should just generate and store the converted list, not the full header.