effects Phase 7: delete the two rebuilt/ fixtures, ExampleStock and ExampleAudioif

[bdbarnett]

lib/audioeffects/rebuilt/examplestock.py and
lib/audioeffects/rebuilt/exampleaudioif.py are fixtures the Phase 2
foundation landed, not effects. Neither NAME is in
audioeffects.__all__, so neither enters audioeffects.ALL and no host can
reach them.

They exist so two things had a subject before any of the 46 was rebuilt:

• tests/test_rebuilt_registry.py — the registry rule resolves a name to a
  file under rebuilt/ and falls back to the old class when there is none.
• tests/test_portability_tier.py — with the audioif-own modules blocked in
  sys.modules, a stock-tier class still builds and renders and an
  audioif-tier class raises its documented ImportError. Without an
  audioif-tier subject that battery passes by having nothing to hold.

At Phase 7, once real rebuilt classes cover both tiers, delete both files
and re-point those two tests at the shipped classes. Check that
tests/test_portability_tier.py still has at least one subject in each tier
after the swap — if it does not, the fixture stays and this issue says why.

Landed on branch effects/p2-foundation, commit b3e1bf5.