core/core: Remove unused header

This isn't used anywhere in either the cpp or header file.

src/core/core.h

@@ -8,7 +8,6 @@
 #include <memory>
 #include <string>
 
-#include <map>
 #include "common/common_types.h"
 #include "core/file_sys/vfs_types.h"
 #include "core/hle/kernel/object.h"