Run unittests with valgrind in compile-unix.sh

When we compile the driver in Evergreen and run the unittests (the tests that don't require a server), it seems like we intended for some configurations to run the unittests with valgrind. Right now it's just a TODO:

https://github.com/mongodb/mongo-c-driver/blob/8d9b6a0968e4427f05ccde7c1c3540e5054cf9c4/.evergreen/compile-unix.sh#L207

Consider actually unittesting with valgrind or, if this is not valuable, remove the traces of valgrind from the unittest scripts and tasks.