chore: remove Julia resources and tests moved to julia-engine extension

These files now live in the julia-engine extension repo at
PumasAI/quarto-julia-engine and are bundled into the extension or
merged into the test tree at CI time.

Removed:
- src/resources/julia/ (bundled into _extensions/julia-engine/)
- tests/smoke/call/engine/julia/ (moved to extension tests/)
- tests/docs/call/engine/julia/ (moved to extension tests/)
- tests/docs/smoke-all/julia/ (moved to extension tests/)
- tests/docs/smoke-all/engine-reordering/julia-engine/ (moved to extension tests/)

Author: jkrumbiegel