firmware: delete in-kernel firmware

The last firmware change for the in-kernel firmware source code was back
in 2013.  Everyone has been relying on the out-of-tree linux-firmware
package for a long long time.

So let's drop it, it's baggage we don't need to keep dragging around
(and having to fix random kbuild issues over time...)

Cc: Kyle McMartin <kyle@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Marek <mmarek@suse.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>