ci: drop toposort/igraph install, no longer needed after #8527

#8527 dropped Optimise's hard toposort dependency in favour of stdlib
graphlib as the fallback backend, so the pip install added here is now
dead weight: nothing in ifcpatch imports toposort any more, and igraph
was always an optional speed-up guarded by pytest.importorskip in
test_Optimise.py, never a hard requirement.

Verified against #8527's merged code (v0.8.0@382f5e0c21) with neither
package installed: all 6 test_Optimise.py tests pass or skip cleanly
(the two igraph-specific tests skip via importorskip instead of
erroring), so the install step served no remaining purpose.

This contribution was produced with the assistance of an AI coding tool.

.github/workflows/ci.yml

@@ -255,7 +255,6 @@ jobs:
           cd ../bsdd && make test || ERROR=1
           pip install deepdiff
           cd ../ifcdiff && make test || ERROR=1
-          pip install toposort igraph
           cd ../ifcpatch && make test || ERROR=1
           pip install -e ../ifc5d --no-deps
           pip install odfpy xlsxwriter