Documentation: remove obsolete networking/multicast.txt file

The original intent of this file was to list limitations in
drivers/hardware relating to multicast use, back when some
modest hardware from the early 1990s did not support things
we might take for granted today.

I was intending to delete some now-gone MCA/token ring entries
in this file, but once I opened it, I found it only contained
information on the earliest (pre-2000) linux networking drivers.

Checking the git history shows that the file hasn't been touched
since 2005.  Clearly nobody is actively consulting this file
as a meaningful reference.

Rather than add a "YES YES YES" line for all of the drivers we
currently have, lets just take advantage of the fact that nobody
is using the file to delete it.

This has the side benefit of not having to do a line-by-line
deletion of the file content as each older driver is expired.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>