Better axis format for timeUnit

[kanitw]

Bug Description

For timeUnit that includes the whole date (date truncation),

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "description": "Google's stock price over time.",
  "data": {"url": "data/stocks.csv"},
  "transform": [{"filter": "datum.symbol==='GOOG'"}],
  "mark": "line",
  "encoding": {
    "x": {
      "timeUnit": "yearmonthdatehoursminutes",
      "field": "date", "type": "temporal"
    },
    "y": {"field": "price", "type": "quantitative"}
  }
}

It's actually generally better if we don't apply timeUnit specifiers here.