TST: Remove unnecessary test images

There's no need to test multiple file formats for tests that:

- are for wrappers that simplify or combine other primitives, such as
  `boxplot`, `eventplot`, `axhspan`, or `fill_between`.
- are for spine or tick settings, such as the `autoscale_tiny_range`,
  `formatter_ticker_*`.
- are for alternate Axes projections, such as polar, Mollweide, or
  skewed Axes.

Author: QuLogic