address_book: hardcode Identity BIP32 path, make TAG_DERIVATION_PATH optional

For Identity contacts, the derivation path is now hardcoded to
m/44'/60'/0'/0/0 as a static const in address_book_crypto.c.
The const path_bip32_t * parameter is removed from all 6 Identity
crypto function signatures; Ledger Account functions are unchanged.

TAG_DERIVATION_PATH is still accepted in TLV payloads (backward
compat) but silently ignored — handlers are no-ops and the tag is
removed from the mandatory-fields check in all 5 Identity flows.

This prepares for the upcoming OS change that drops bip32_path from
the HMAC syscalls entirely.