NetworkPkg/HttpDxe: Destroy the TLS instance when cleaning up the HTTP child

During clean up the HTTP child, all resources used by it should be cleaned. But
currently, TLS instance is not destroyed.

This patch is to fix this issue.

Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>