cleanup: Remove redundant `-DSODIUM_EXPORT` from definitions. #3652

Job	Run time
docker-toxcore-js	2m 27s
docker-clusterfuzz	36s
docker-bootstrap-node	1m 46s
docker-esp32	2m 8s
docker-fuzzer	8s
docker-win64	17s
docker-win32	8s
docker-bootstrap-node-websocket	11s
	7m 41s

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