Remove unused classes and stray headers

Found by reviewing headers with 1 or less matching includes:
```
find -name thirdparty -prune -o -name "*.h" -exec basename {} \; | sort -u > headers
for header in $(cat headers); do echo "$header: "; rg -l "#include \"(.*/)?$header\"" | wc -l; done > list-includes
```

main/tests/test_oa_hash_map.cpp

@@ -30,9 +30,8 @@
 
 #include "test_oa_hash_map.h"
 
-#include "core/os/os.h"
-
 #include "core/oa_hash_map.h"
+#include "core/os/os.h"
 
 namespace TestOAHashMap {