Make breaking channel hierarchy changes (Discord4J#960)

* Modify channel hierarchy in preparation for threads

* Use 'TopLevel' names instead of temp names, remove duplicate docs

* Add deprecation docs

* Add class docs

* Make CategorizableChannel extend TopLevelGuildChannel

* Make breaking channel hierarchy changes

* (Re)move methods deprecated in preparation for threads
* Move most base channel method implementations to default methods
* Remove most base channel classes

* Don't deprecate GuildChannel#getEffectivePermissions

* Remove BaseTopLevelGuildChannel, add license headers, docs

* Remove unused imports

* Remove unused imports

Co-authored-by: quanticc <abarcab.ivan@gmail.com>

core/src/main/java/discord4j/core/object/entity/channel/BaseChannel.java

@@ -16,25 +16,24 @@
  */
 package discord4j.core.object.entity.channel;
 
-import discord4j.common.util.Snowflake;
-import discord4j.core.GatewayDiscordClient;
 import discord4j.core.util.EntityUtil;
 import discord4j.discordjson.json.ChannelData;
+import discord4j.core.GatewayDiscordClient;
+import discord4j.common.util.Snowflake;
 import discord4j.rest.entity.RestChannel;
-import reactor.core.publisher.Mono;
 import reactor.util.annotation.Nullable;
 
 import java.util.Objects;
 
 /** An internal implementation of {@link Channel} designed to streamline inheritance. */
 class BaseChannel implements Channel {
 
-    /** The raw data as represented by Discord. */
-    private final ChannelData data;
-
     /** The gateway associated to this object. */
     private final GatewayDiscordClient gateway;
 
+    /** The raw data as represented by Discord. */
+    private final ChannelData data;
+
     /** A handle to execute REST API operations for this entity. */
     private final RestChannel rest;
 
@@ -56,31 +55,12 @@ public final GatewayDiscordClient getClient() {
     }
 
     @Override
-    public final Snowflake getId() {
-        return Snowflake.of(data.id());
-    }
-
-    @Override
-    public RestChannel getRestChannel() {
+    public final RestChannel getRestChannel() {
         return rest;
     }
 
     @Override
-    public final Type getType() {
-        return Type.of(data.type());
-    }
-
-    @Override
-    public final Mono<Void> delete(@Nullable final String reason) {
-        return rest.delete(reason);
-    }
-
-    /**
-     * Gets the raw data as represented by Discord.
-     *
-     * @return The raw data as represented by Discord.
-     */
-    ChannelData getData() {
+    public final ChannelData getData() {
         return data;
     }