Make many Parser fields and methods private again

They were made protected in 2017 and used for a short time by our
B parser, but this was quickly replaced with a different solution - see:
hhu-stups/probparsers@5fc63a6cb8caaff3d567ebfee1d706230cea00f6

It seems that they are not used anywhere else, so make them private
again so nobody gets any ideas...