tools: bpftool: remove duplicated error message on prog load

do_pin_fd() will already print out an error message if something
goes wrong.  Printing another error is unnecessary and will break
JSON output, since error messages are full objects:

$ bpftool -jp prog load tracex1_kern.o /sys/fs/bpf/a
{
    "error": "can't pin the object (/sys/fs/bpf/a): File exists"
},{
    "error": "failed to pin program"
}

Fixes: 49a086c2 ("bpftool: implement prog load command")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/prog.c

@@ -694,10 +694,8 @@ static int do_load(int argc, char **argv)
 		return -1;
 	}
 
-	if (do_pin_fd(prog_fd, argv[1])) {
-		p_err("failed to pin program");
+	if (do_pin_fd(prog_fd, argv[1]))
 		return -1;
-	}
 
 	if (json_output)
 		jsonw_null(json_wtr);