bugfix: remove export keyword for locally used declaration

[kazushisan]

export const a = 'a';

export const b = 'b';

export const c = 'c';

console.log(b);

when c is the only used export within the project, the expected behavior is

const b = 'b';

export const c = 'c';

console.log(b);

however the current behavior is

export const b = 'b';

export const c = 'c';

console.log(b);

This PR will fix this bug to remove export keywords for locally used declarations.