Remove SQLiteDatabase::interrupt and related code

it's not used from anywhere, but calls WTF::yield() which doesn't work
with WebThreads

BUG=none
R=mkwst@chromium.org, michaeln@chromium.org

Review URL: https://codereview.chromium.org/593603003

git-svn-id: svn://svn.chromium.org/blink/trunk@182592 bbb929c8-8fbe-4397-9dbb-9b2b20218538