refactor: remove dead code

Author: rhargreaves

src/bpf_program.cpp

@@ -53,5 +53,4 @@ void BpfProgram::attach_xdp(int ifindex, const std::string& interface)
     if (!link) {
         throw BpfError("Failed to attach XDP program to interface " + interface);
     }
-    interface_name = interface;
 }

src/bpf_program.hpp

@@ -7,7 +7,6 @@ class BpfProgram {
 private:
     knock_bpf* skel = nullptr;
     bpf_link* link = nullptr;
-    std::string interface_name;
 
 public:
     explicit BpfProgram(const knock_config& config);