Remove singleton notations for tuples

Remove singleton notations {[ x,y ]} and {[ x,y,z ]} for {[ (x,y) ]} and {[ (x,y,z) ]}. They date back to the time we used the singleton class with a product for maps (there's now the singletonM class).

LGTM.

merged

mentioned in commit 6d001c9d

@iris-users This is a breaking change.

Note that this broke Iris, see iris@5011fae3

Turns out the notation was used twice by accident (whereas other singletons in that file just used the tuple explicitly).