Remove parallel mode from SplitStream

- [x] Remove parallel mode support from [SplitStream](https://github.com/51Degrees/Pearl/blob/feature/api-file-cache-4-2/API/Distributor/Distributor/Helpers/SplitStream.cs).
  - [x] `useParallel` constructor parameter
  - [x] `_useParallel` i-var.
  - [x] if-branches.
  - [x] `syncLock` and lock statement.
- [x] Check use sites.
- [x] Update tests.