[usercase-webapi] Fix a removing Bookmark issue in webapi-usercase-tizen-tests.
The current logic of remove() is wrong which is trying to remove a new bookmark object.
Instead, remove() should remove an existing bookmark which must be already added.

BUG=XWALK-2073
Merge pull request #544 from yejingfu/bug-fix

[usercase-webapi] Fix a removing Bookmark issue in webapi-usercase-tizen...