fix: remove redundant type checks flagged by compiler

[gilbertwong96]

The compiler emitted two warnings for checks that are always true or always false due to prior type narrowing.

In Spitfire.Env.expand/3, the :import clause re-checked is_atom(arg) inside a with despite the enclosing if is_atom(arg) do already narrowing arg to atom(). Dropped the redundant guard, matching the sibling :require clause.

In Spitfire.skip_call_args/2, the first cond clause (token == nil or token == :eof) already handles :eof, so the trailing token == :eof in the second clause compared disjoint types and was dead. Removed it.

[mhanberg]

Please revert the 1.20/otp29 change.

The parser tests for equality against the core parser and that changes between versions so we only test against the latest version and I haven't made the updates for 1.20 yet.