logging: Minimal mode configuration cleanup

Remove LOG_MINIMAL kconfig option which was confusing
since LOG_MODE_MINIMAL existed. LOG_MINIMAL was used to
force minimal mode but because of invalid dependencies
it was leading to issues.

Refactored code to use LOG_MODE_MINIMAL everywhere and
renamed LOG_MINIMAL to LOG_DEFAULT_MINIMAL which has impact
on defualt logging mode (which still can be later changed
in conf file or in menuconfig).

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>