[no-vary-search-hint] Remove empty sets of nvs hint candidates

Small follow up to crrev.com/c/4539325. Remove empty sets of
no vary search hint candidates from
`no_vary_search_hint_on_standby_candidates_` when removing a candidate.

Bug: 1416659
Change-Id: Iaae5c3e8527a1111dd1e5d349068b4f99cc2da73
Disable-Rts: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4557268


Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Reviewed-by: Iman Saboori <isaboori@google.com>
Reviewed-by: Simon Pelchat <spelchat@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148693}