refactor: remove unused files; improve build UX #6226

Job	Run time
get-grouped-hardware	9s
get-changed-files	3s
core-coverage	11s
board-changes	8s
nightly	0s
compile-matrix	2s
build (nice_nano_v2, [{"shield":"corne_left"},{"shield":"corne_right"},{"shield":"romac"},{"shiel...	4m 46s
build (nrfmicro_13, [{"shield":"corne_left"},{"shield":"corne_right"},{"shield":"romac"},{"shield...	3m 8s
build (proton_c, [{"shield":"corne_left"},{"shield":"corne_right"},{"shield":"romac"},{"shield":"...	2m 15s
build (bdn9_rev2, [{}])	1m 31s
build (nice60, [{}])	1m 43s
build (seeeduino_xiao_ble, [{"shield":"hummingbird"}])	1m 37s
build (nrf52840_m2, [{"shield":"m60"}])	1m 45s
build (planck_rev6, [{}])	1m 32s
build (nice_nano, [{"shield":"romac_plus","cmake-args":"-DCONFIG_ZMK_RGB_UNDERGLOW=y -DCONFIG_WS2...	1m 47s
	20m 37s

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