pw_multisink: Remove deprecated GetEntry
pw_multisink: Remove deprecated GetEntry

Use PopEntry instead of GetEntry.

No-Docs-Update-Reason: deprecated API.
Change-Id: Ifa82ec1f167fe9dcd50a53c568b2fba86d7a44ec
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/59620
Commit-Queue: Carlos Chinchilla <cdf38d65522bb1d68fb5b958430f926735c310be@google.com>
Pigweed-Auto-Submit: Carlos Chinchilla <cdf38d65522bb1d68fb5b958430f926735c310be@google.com>
Reviewed-by: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed