boards: drop the unsigned mcuboot default on espressif socs

Boards built around Espressif SoCs copied the vendor board
fragment that selects BOOT_SIGNATURE_TYPE_NONE. Remove it so they
follow the same RSA default as the Espressif boards.

Assisted-by: Claude:opus-5
Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>