is a library callback function that unregisters a library expression manager with name mgr.
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Used in C/C++ code in a library function called from Mathematica, typically in library uninitialization functions WolframLibrary_uninitialize.
A nonzero error code value will be returned if manager mgr is not currently registered.
When the manager mgr registered by registerLibraryExpressionManager(mgr, manageFun) unregistered, manageFun will be called with mode 1 for all values of id that have been generated by evaluating that have not already been released.