Revert "Delete webpage asynchronously when exiting."

This reverts commit 95fa882b00b3fea34625ea9118a4a607d490cdfe.
Stop javascript processing before deleting web pages.

Otherwise, a simple example such as this would produce unexpected
output:

console.log("Hello World!");
phantom.exit();
console.log("Meh, noone should see me!");

Note thought that a direct deletion of the webpage results in a crash
in Qt5, as apparently the javaScript evaluation is still accessing
page contents when we return from our Phantom::exit() call.