Support removal of assert variable assignment

[twada]

Support removal of assignment to assert variable. (For example, CoffeeScript produces this style of assinment) On the other hand, do not remove VariableDeclarator since there are too many assumptions to make.

before:

var assert;
assert = require('assert');
function add (a, b) {
    console.assert(typeof a === 'number');
    assert(!isNaN(a));
    assert.equal(typeof b, 'number');
    assert.ok(!isNaN(b));
    return a + b;
}

after:

var assert;
function add(a, b) {
    return a + b;
}