Remove the top-level DebugDriverThread in ProcessWindows.

Originally the idea was that we would queue requests to a master
thread that would dispatch them to other slave threads each
responsible for debugging an individual process.  This might make
some scenarios more scalable and responsive, but for now it seems
to be unwarranted complexity for no observable benefit.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@221561 91177308-0d34-0410-b5e6-96231b3b80d8