Delete the skp_shader_warmup_test harness if possible

[dnfield]

This affects Fuchsia. It was added in flutter/engine#19260. It's not clear to me whether Fuchsia still needs this or not (@akbiggs may know).

Right now, it is the only part of the codebase that still needs PictureLayer after dropping the flag for display list.

[dnfield]

Chatted with a couple Fuchsia folks and it seems like there's likely coverage downstream - it's not clear to me what this coverage is really protecting anyway, so just going to remove it.

[dnfield]

For

[dnfield]

For future me: the test harness was using picture layers, but the acutal impl just directly interacts with SkSurfaces it creates: https://github.com/flutter/engine/blob/e986f43abf1096f5a5af3270c134d17eb9b18dd9/shell/platform/fuchsia/flutter/engine.cc#L924-L954

If we really need a test harness for this, we should add one back that's fuchsia specific and uses the same method as Fuchsia.

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