test: remove css_aspect_ratio.rs (fully migrated to HTML cases)

All 14 aspect-ratio tests (3 assert_xml! + 11 code-constructed) are now
HTML cases in tests/cases/custom_css_aspect_ratio/. The .rs file had
code-constructed tests using Node API (set_aspect_ratio, set_box_sizing
etc.) which all map to CSS properties — translated manually.

Author: TtTRz