Revert "Fix tooltips not visible for NVD3 charts on Firefox (#7822)"

[etr2460]

Reverts #7929

This broke embedded charts. I tested by creating an index.html file with the following code:

<iframe width="600" height="400" seamless frameBorder="0" scrolling="no"
  src="http://localhost:5000/superset/explore/?form_data=%7B%22datasource%22%3A%222__table%22%2C%22viz_type%22%3A%22table%22%2C%22slice_id%22%3A157%2C%22url_params%22%3A%7B%7D%2C%22granularity_sqla%22%3A%22year%22%2C%22time_grain_sqla%22%3A%22P1D%22%2C%22time_range%22%3A%222014-01-01+%3A+2014-01-02%22%2C%22groupby%22%3A%5B%22country_name%22%5D%2C%22metrics%22%3A%5B%22sum__SP_POP_TOTL%22%5D%2C%22percent_metrics%22%3A%5B%5D%2C%22timeseries_limit_metric%22%3Anull%2C%22row_limit%22%3A50000%2C%22include_time%22%3Afalse%2C%22order_desc%22%3Atrue%2C%22all_columns%22%3A%5B%5D%2C%22order_by_cols%22%3A%5B%5D%2C%22adhoc_filters%22%3A%5B%5D%2C%22table_timestamp_format%22%3A%22%25Y-%25m-%25d+%25H%3A%25M%3A%25S%22%2C%22page_length%22%3A0%2C%22include_search%22%3Afalse%2C%22table_filter%22%3Afalse%2C%22align_pn%22%3Afalse%2C%22color_pn%22%3Atrue%7D&standalone=true&height=400">
</iframe>

Before revert:

After revert:

@schoel-bis @graceguo-supercat @mistercrunch