No longer reject dictionaries with literals maxSymbolValue < 255

No longer reject dictionaries with literals maxSymbolValue < 255

We already have logic in our Huffman encoder to validate Huffman tables with missing symbols.
We use this for higher compression levels to re-use the previous blocks statistics, or when the dictionaries table has zero-weighted symbols.
This check was leftover as an oversight from before we added validation for Huffman tables.

I validated that the `dictionary_loader` fuzzer has coverage of every line in the `ZSTD_loadCEntropy()` function to validate that it is correctly testing this function.