Hardcoding _ITERATOR_DEBUG_LEVEL causes linker issues in MSVC debug builds

The code https://github.com/MariaDB/server/blob/32980c6b94a8481cad2d322366559c5e9090feb7/storage/rocksdb/CMakeLists.txt#L283-L284 does:
```

1. Some checks in C++ runtime that make debug build much slower
   ADD_DEFINITIONS(-D_ITERATOR_DEBUG_LEVEL=0)
   ```
   but this is causing issues in our builds when producing MSVC builds with debug configurations such as:
   ```
   snappy.lib(snappy.cc.obj) : error LNK2038: mismatch detected for '_ITERATOR_DEBUG_LEVEL': value '2' doesn't match value '0' in sst_dump.cc.obj
   ```

I know that this is done intentionally to improve rocksdb debug build performance, so I understand if you close this as Won't Fix. But I am attaching a patch which would remove this check in order to make debug builds compile more reliably. Please let me know if you would like me to create a PR with this patch.