Remove BigInteger for C# pidigits

[jjxtra]

C# BigInteger is known to be poorly implemented for performance, so have removed it to give a better showing of the compiler with raw math

[jjxtra]

Let me know if you want this in a new file instead

[hanabi1224]

Thanks for the new version, however, it breaks comparability to other implementations. For this problem, all implementations should use a general-purpose big integer library. And in this case, it's more of evaluating the ecosystem than the compiler itself