ls-remote: leakfix for not clearing server_options

ls-remote: leakfix for not clearing server_options

Ensure `server_options` is properly cleared using `string_list_clear()`
in `builtin/ls-remote.c:cmd_ls_remote`.

Although we cannot yet enable `TEST_PASSES_SANITIZE_LEAK=true` for
`t/t5702-protocol-v2.sh` due to other existing leaks, this fix ensures
that "git-ls-remote" related server options tests pass the sanitize leak
check:

  ...
  ok 12 - server-options are sent when using ls-remote
  ok 13 - server-options from configuration are used by ls-remote
  ...

Signed-off-by: Xing Xin <xingxin.xx@bytedance.com>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>