[bitcoin/bitcoin] Refactoring CRPCCommand with enum category (#13945)

@isghe pushed 4 commits.

fa6d149 remove unnecessary comments, and false instead of 0
3a61034 std::string category to RPCCategory category
d8e5619 remove src/rpc/server.cpp*to_upper
7f99798 class CRPCCommand to struct CRPCCommand