[Service] Refactor unload xwalk extension

[Service] Refactor unload xwalk extension

unloadInstance() is introduced for optimizing memory on global
model. But with irregular crash problems on UvTaskRunner's
OnTimeout function, it try to access xwalk instance after deleted.

This patch will remove calling unloadInstance() API.
Unloading xwalk extension will be refactored below patch.

Related patch:
  https://review.tizen.org/gerrit/246586/

Change-Id: Ie6a74b815680f02530fd2553427d03bf1634d612
Signed-off-by: DongHyun Song <dh81.song@samsung.com>