ext: Kconfig: Remove redundant 'default n' properties

Bool symbols implicitly default to 'n'.

A 'default n' could make sense e.g. in a Kconfig.defconfig file, if you
wanted to override a 'default y' on the base definition of the symbol,
but it doesn't seem to be used like that on any of these symbols.

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