Remove partial key size limit

[cheme]

The pr paritytech/substrate#12566 remove the partial key lenght limit as it was incorrectly implemented and did not make sense (code try to truncate is unpractical).
A limit should be added but as a max key length for host function.
In trie partial key description, the partial key could therefore be remove (of length $0 <= l_("pk"_N) < 2^(16)$ in nibbles`