build: remove double LIBBITCOIN_SERVER from bench-Makefile
build: remove double LIBBITCOIN_SERVER from bench-Makefile

Former-commit-id: 9e48b0fb4e568d5350406b053fe5032f273bf13d
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin