VideoCommon: Deinit Graphics Mod Manager explicitly

[noahpistilli]

g_graphics_mod_manager wasn't shutdown in VideoBackendBase::ShutdownShared causing

libc++abi.dylib: terminating with uncaught exception of type std::__1::system_error: mutex lock failed: Invalid argument

on macOS.

The cause of the error was EventHook in GraphicsModManager calling it's deconstructor, by which time the mutex in the static Storage was already destroyed.