Winter drop feature flag is not added to the removed features list

Previous experimental features, when graduating out of experimental gameplay, would have a data fixer added to move the feature flag from enabled_features to removed_features in level.dat. This same treatment does not apply to the minecraft:winter_drop feature flag; instead, the flag is simply forgotten when loaded in a version with the flag removed. This seems to be an unintentional inconsistency as this feature flag doesn't seem to have any special characteristics compared to other feature flags.

Steps to reproduce:

1. Create a world in Minecraft 1.21.3 with the winter drop experiment enabled
2. Upgrade the world to Minecraft snapshot 24w46a
3. Crash the game manually
4. Observe the list of removed features does not mention minecraft:winter_drop 

The impact this issue has on gameplay is that worlds that had the minecraft::winter_drop feature flag removed will not show that fact in the Removed feature flags entry of crash reports. Again, this is inconsistent with the previous removed feature flags (minecraft:update_1_20, minecraft:update_1_21, and minecraft:bundle).