Remove unnecessary regression tests

[RunDevelopment]

With #2829, a few old tests became unnecessary. The reference test verifies that all images in tests/images decode without panic or error, and have the expected dimensions and contents according to their reference images. So any test that only checks a subset of these properties is unnecessary.

Changes:

• Remove ICO regression test. This one tested that bmp_v5_with_icc.ico decodes without error and has the expected dimensions.
• Remove WebP regression test. This one tested that WebP animations decode without error and with the expected number of frames. This is implicitly tested by the reference test.