misc: 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 on any of these symbols though.

Piggyback removal of some 'default ""' properties on string symbols. The
empty string is the default.

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