Remove unused code

- ReadLocker constructors that take a lock
- Unconditional Lock::ReadLock and ReadLocker::Lock
  (all consumers use TryLock)
- Make Unlock protected, as it has no external consumers


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@187147 91177308-0d34-0410-b5e6-96231b3b80d8