boards: Kconfig: Remove redundant 'default n' properties

Bool symbols implicitly default to 'n'.

A 'default n' can make sense e.g. in a Kconfig.defconfig file, if you
want to override a 'default y' on the base definition of the symbol. It
isn't used like that for any of the removed properties though.

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