net/dcb: make dcbnl.c explicitly non-modular

The Kconfig currently controlling compilation of this code is:

net/dcb/Kconfig:config DCB
net/dcb/Kconfig:        bool "Data Center Bridging support"

...meaning that it currently is not being built as a module by anyone.

Lets remove the modular code that is essentially orphaned, so that
when reading the driver there is no doubt it is builtin-only.

Since module_init translates to device_initcall in the non-modular
case, the init ordering remains unchanged with this commit.  We can
change to one of the other priority initcalls (subsys?) at any later
date, if desired.

We also delete the MODULE_LICENSE tag etc. since all that information
is (or is now) already contained at the top of the file in the comments.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Anish Bhatt <anish@chelsio.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: Shani Michaeli <shanim@mellanox.com>
Cc: netdev@vger.kernel.org
Signed-off-by: Paul...