live-preview: Remove dead code related to design mode #18170

Job	Run time
android	13s
cpp_cmake (macos-13, 1.77)	10s
cpp_test_driver (ubuntu-22.04)	14s
cpp_cmake (windows-2022, nightly)	8s
build_and_test (ubuntu-22.04, 1.77)	16s
cpp_cmake (ubuntu-22.04, stable)	15s
cpp_test_driver (macos-13)	12s
build_and_test (macos-13, stable)	12s
esp-idf-quick	12s
cpp_test_driver (windows-2022)	9s
build_and_test (windows-2022, stable)	11s
build_and_test (windows-2022, 1.77)	14s
ffi_32bit_build	11s
build_and_test (windows-2022, beta, --exclude ffmpeg --exclude gstreamer-player)	11s
fmt_test	14s
build_and_test (ubuntu-22.04, nightly)	17s
mcu (pico-st7789, thumbv6m-none-eabi)	24s
mcu_esp	10s
mcu (stm32h735g, thumbv7em-none-eabihf)	11s
miri	13s
node_test (ubuntu-22.04)	1s
node_test (macos-13)	1s
python_test (ubuntu-22.04)	1s
updater_test (0.3.0)	1s
node_test (windows-2022)	1s
python_test (macos-13)	1s
vsce_build_test	1s
python_test (windows-2022)	1s
docs / docs	1s
format_lint_typecheck / format_lint_typecheck	1s
tree-sitter / tree-sitter-tests	1s
wasm / wasm	1s
wasm_demo / wasm_demo	1s
wasm / build-slintpad	1s
cpp_package_test	1s
	4m 32s

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