Fixed an assertion which did not hold when the debugger was recursively entered.

Removed the clearing of the preemption flag in debugger entry as there was no need for it. The handling of preemption by the stack guard takes care of the in debugger situation anyway.
Review URL: http://codereview.chromium.org/48004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1520 ce2b1a6d-e550-0410-aec6-3dcde31c8c00