Remove unused entities from language-snippets.ent

Audit found 83 ENTITY definitions that are not referenced anywhere
in the repo, neither directly (via &name;) nor transitively through
other used entities. Most fall into a few categories:

- Deprecation banners for PHP 5.x / 6.x / 7.x cycles whose target
  functions have been removed for years
- Snippets for removed extensions (mcrypt, mssql, mcve, dbm, sqlite,
  t1lib, safemode, ...)
- Stale "example.outputs.*" / "examples.outputs.*" helpers
- Orphaned imagick / mongodb / gd / haru / tidy notes

No behavior change; only dead declarations are removed.

Author: lacatoire