Remove redundant test in TestExitDuringStep

After recent changes, test_thread_state_is_stopped has become equivalent to test_step_in, as the
function exit_during_step_base was not using the "test_thread_state" parameter. As test was
XFAILed on all platforms anyway, and we have other tests for the bug which it (used to) test, I
am simply removing the function.

llvm-svn: 259517