Revert "Added a properties to check if board/platofrm supports encryption"

This reverts commit 16810f58fe4d381a2cbc0a19da8d304fce93f56f.

At the moment this is not needed since we support only a key type. We could add it in later if a real need arises for other key types.

Author: umbynos

boards.txt

@@ -81,7 +81,6 @@ envie_m7.build.header_size=0x20000
 envie_m7.build.alignment=32
 envie_m7.build.version=1.2.3+4
 envie_m7.menu.security.sien.build.postbuild.cmd="{tools.imgtool.path}/{tools.imgtool.cmd}" {tools.imgtool.build.pattern}
-envie_m7.menu.security.sien.build.keys.type=public_keys
 envie_m7.menu.security.sien.build.keys.keychain={runtime.hardware.path}/mbed/libraries/MCUboot/default_keys
 envie_m7.menu.security.sien.build.keys.sign_key=ecdsa-p256-signing-key.pem
 envie_m7.menu.security.sien.build.keys.encrypt_key=ecdsa-p256-encrypt-key.pem