Remove extra symbols in `symbols/de` that are not defined in DIN 2137-01:2018-12.

I found a couple of codes with symbols, that are not defined in DIN 2137-01:2018-12.

I suggest to leave them in NoSymbol state. It eases future adding of new symbols when the norm develops further and avoids trouble.

@mshugo: thank you for your good work! I was waiting for this driver a long time.