Add Checkstyle check for unused imports

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/legacy/CraftLegacy.java

@@ -9,7 +9,6 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
 import net.minecraft.SharedConstants;
-import net.minecraft.core.IRegistry;
 import net.minecraft.core.registries.BuiltInRegistries;
 import net.minecraft.nbt.DynamicOpsNBT;
 import net.minecraft.nbt.NBTBase;