Remove redundant baseline tests in test_image.

... in prevision for the "stop renormalizing to 0-1 prior to resampling"
patch, which will lead to some baseline changes.

This removes
- test_image_interps (testing interpolations); subsumed by
  test_imshow_masked_interpolation (which tests more interpolations and
  also masking).
- test_imshow (testing the extent kwarg, despite the unclear name of the
  test -- see the original commit, 387fb64); subsumed by more complex
  tests that also check the extent kwarg, e.g. test_image_cliprect and
  test_rotate_image.

Author: anntzer