[CIR] Remove also the !cir.void return type from the textual IR

[keryell]

This is the second phase of #1203, as the first phase has demonstrated that fixing the IR to avoid breaking an MLIR invariant can work.
Now, to avoid adding a new feature in the MLIR parser infrastructure, we change the way CIR IR textual representation is done by removing also !void or !cir.void in the function return types to just follow the normal way the other dialects work to represent function types.
See #1203 (comment) for more context.