core: remove refcount for bpf program

Currently ref count of bpf-program is kept in user space. However, the
kernel already implements its own ref count. Thus the ref count we keep for
bpf-program is redundant.

This PR removes ref count for bpf program as part of a task to simplify
bpf-program and remove redundancies, which will make the switch to
code-compiled BPF programs easier.

Part of #19270

src/core/unit.c

@@ -770,7 +770,7 @@ Unit* unit_free(Unit *u) {
 
         hashmap_free(u->bpf_foreign_by_key);
 
-        bpf_program_unref(u->bpf_device_control_installed);
+        bpf_program_free(u->bpf_device_control_installed);
 
 #if BPF_FRAMEWORK
         bpf_link_free(u->restrict_ifaces_ingress_bpf_link);