unused variable disappear

[listenwhy]

Describe the bug

Reproduction Steps

fn main() {
println('hello vlang')
a := 'aa'
}

Expected Behavior

unused variable disappear.
fix it

Current Behavior

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.1 d93488b

Environment details (OS name and version, etc.)

windows

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