mkeficapsule: Remove dtb related options

commit 322c813f ("mkeficapsule: Add support for embedding public key in a dtb")
added a bunch of options enabling the addition of the capsule public key
in a dtb.  Since now we embedded the key in U-Boot's .rodata we don't this
this functionality anymore

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>