minifier: remove assignment to unread variable

[sapphi-red]

let foo = {}
export function _setFoo(bar) {
  console.log('foo')
  foo = bar
}

can be replaced with

let foo = {}
export function _setFoo(bar) {
  console.log('foo')
}

because foo is not read by anything.
playground

[sapphi-red]

implemented by #11796

[Boshen]

This is reverted.

[sapphi-red]

implemented by #12509