boards: arduino_101_mcuboot: Remove Kconfig settings moved to DTS

The PHYS_LOAD_ADDR Kconfig symbol was removed by commit b8ea7c88
("x86: remove HAS_DTS checking"). The ROM_SIZE Kconfig symbol was
removed by commit e524f0b8 ("dts: x86: derive RAM and ROM size from
dts instead of Kconfig").

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>