media: cec: extron-da-hd-4k-plus: drop external-module make commands

Delete the external-module style Makefile commands. They are not needed
for in-tree modules.

This is the only Makefile in the kernel tree (aside from tools/ and
samples/) that uses this Makefile style.

The same files are built with or without this patch.

Fixes: 056f2821 ("media: cec: extron-da-hd-4k-plus: add the Extron DA HD 4K Plus CEC driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>