Mbed: Remove unused header size config

            "help": "Size of the scratch area, in bytes. If needed, please set on a per-target basis.",

            "macro_name": "MCUBOOT_SCRATCH_SIZE"

        },

        "header-size": {

            "help": "Header size, in bytes, prepended to the bootable application image. Should be one or multiple times the sector size.",

            "macro_name": "MCUBOOT_HEADER_SIZE",

            "required": true,

            "value": 4096

        },

        "validate-primary-slot": {

            "help": "Always check the signature of the image in the primary slot before booting, even if no upgrade was performed. This is recommended if the boot time penalty is acceptable.",

            "macro_name": "MCUBOOT_VALIDATE_PRIMARY_SLOT",