Move deleting thread on TARGET_WAITKIND_THREAD_EXITED to core

Move deleting thread on TARGET_WAITKIND_THREAD_EXITED to core

Currently, infrun assumes that when TARGET_WAITKIND_THREAD_EXITED is
reported, the corresponding GDB thread has already been removed from
the GDB thread list.

Later in the series, that will no longer work, as infrun will need to
refer to the thread's thread_info when it processes
TARGET_WAITKIND_THREAD_EXITED.

As preparation, this patch makes deleting the GDB thread
responsibility of infrun, instead of the target.

Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: I013d87f61ffc9aaca49f0d6ce2a43e3ea69274de