Disable circular reference collection to improve autocompletion performance

This brings AutocompletionPerformanceTest::testProvideAllFromStubs down
from ~60 ms to ~20 ms since commit
867769f1.

Sources:
  - https://scrutinizer-ci.com/blog/composer-gc-performance-who-is-affected-too
  - https://blog.ircmaxell.com/2014/12/what-about-garbage.html

References #43