Use the value receiver instead of the pointer receiver in Pubkey.Identity() (Triage on ostracon#492)

[0Tech]

The subject of the triage:

• Use the value receiver instead of the pointer receiver in Pubkey.Identity() ostracon#492

[Mdaiki0730]

It seems that crypto/composite/composite.go does not exist at the stage of v0.38.0.
https://github.com/cometbft/cometbft/tree/v0.38.0/crypto

crypto/composite/composite.go was experimentally introduced in ostracon#194 for BLS signatures and removed in ostracon#541.
Finschia/ostracon#194
Finschia/ostracon#541

It seems that there is no need to reflect it in cometbft.