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/test/test-bpf-foreign-programs.c

@@ -155,7 +155,7 @@ static int pin_programs(Unit *u, CGroupContext *cc, const Test *test_suite, size
         assert_se(paths_ret);
 
         for (size_t i = 0; i < test_suite_size; i++) {
-                _cleanup_(bpf_program_unrefp) BPFProgram *prog = NULL;
+                _cleanup_(bpf_program_freep) BPFProgram *prog = NULL;
                 _cleanup_free_ char *str = NULL;
 
                 r = bpf_foreign_test_to_string(test_suite[i].attach_type, test_suite[i].bpffs_path, &str);