packages: remove the last remaining copyright notices

There is no real reason to keep copyright notices in just four
packages, while none of the other packages have such copyright
notices.

The license is already clearly announced by the COPYING file in the
top Buildroot source directory. The authors are clearly credited
through the Git history of the project.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>