configs/raspberrypi: remove BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTBS

BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTBS became ineffective in commit a5ce7590
(package/rpi-firmware: bump version), and was removed in commit 4e92ffdc
(rpi-firmware: remove no longer used BR2_PACKAGE_RPI_FIRMWARE_INSTALL_DTBS).
Remove it from the Pi configs.

Also, remove the now obsolete comment in these config files.

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>