chore(assets): remove '* Temp (LTR|RTL)' staging folders + detector allowlist

These 20 folders were staging artifacts that caused non-deterministic React
atom/font output via filename collisions in importer/generate.js (see #1086).
They contain SVGs named after the OPPOSITE icon (e.g. 'Arrow Undo Temp RTL/
SVG/ic_fluent_arrow_redo_*.svg'), have no metadata.json, and are not consumed
by any active pipeline.

With the folders gone, the KNOWN_DUPLICATE_SOURCE allowlist in
importer/generate.js is also removed; the default --on-duplicate=fail now
guards against any new colliding asset folders.

Verified: two clean 'nx run react-icons:build' runs after this change
produce byte-identical lib/, lib-cjs/, and metadata.json.

Follow-up to #1086.

Author: Hotell