fix: use builtin_initialize in library_search

[joehendrix]

This replaces a few uses of initialize with builtin_initialize, and removes some unneeded functionality added when it was unclear if lazy discriminator trees would be efficient enough.

[leanprover-community-mathlib4-bot]

Mathlib CI status (docs):

• ❗ Std/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 995726f75fd7eed223c2189a54f6df3293185327 --onto 317adf42e92a4dbe07295bc1f0429b61bb8079fe. (2024-03-14 07:11:54)