CodeQL: Make any() expression and exists() formula variable identifier optional

What do you think about making the variable identifiers of the CodeQL [`any(...)` expression](https://codeql.github.com/docs/ql-language-reference/expressions/#any) and [`exists(...)` formula](https://codeql.github.com/docs/ql-language-reference/formulas/#exists) optional if they do not have any formulas?
Currently the CodeQL language specification requires an identifier even though it is not used.

Examples:
```ql
exists(GadgetClass unused) // Check whether a vulnerable "gadget" class exists on the class path
and any(CustomMethodCall unused).getArgument(0) instanceof CustomArgument
```
Here in both cases it is currently necessary to specify a variable identifier (`unused`), even though it is not used.

For the `exists` formula this could lead to some ambiguity because it currently allows using expressions (e.g. `exists(call.getAnArgument())`), however because type names as part of variable declarations cannot contain a period, this should be unambiguous.