These functions operate on whole series, not on samples, so they do not
fit into the table of functions that return a Vector. Remove the stub
entries that were left to help downstream users of the code identify
what changed.
We cannot remove the entries from the `FunctionCalls` map without
breaking `TestFunctionList`, so put some nils in to keep it happy.
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>