Generate realms.cfg, drop unreferenced config entries.

R=rmistry@google.com
BUG=skia:12502

Change-Id: I6762f35ca3fc8f9afc261017e0ae13ac9fa26135
Reviewed-on: https://skia-review.googlesource.com/c/lottie-ci/+/455739
Auto-Submit: Vadim Shtayura <vadimsh@google.com>
Reviewed-by: Ravi Mistry <rmistry@google.com>